*Pedantic clarification
====
The GPL lacks the ability to force someone to provide source to their customers. Only the courts have that ability. While the GNU Organisation and others may WANT "us" to give source to our customers*1, this is not what we currently forced to do. The LGPL terms can also be used by the courts to force 'us' to make source available to customers as well. For the platform, say everything in the libraries/joomla, libraries/legacy, and libraries/compat must be available.
f you modify code in those directories, those changes must be available. So you can't change/add features to existing code without making those changes available.
*Factual Rebuttal/Historical Detail
====
"force us to make source available" - which source? All of it? Nope - all legal precedent is against this. I keep coming back to small embedded systems because they are easy examples. The Mattel/Emsoft Juicebox was a cool little mp3/video player for kids
http://www.elinux.org/JuiceBoxI picked up over 20 of them for 5 bucks each when they were discontinued because their neat to hack on.
The Juicebox runs a custom install of Linux[GPL] and can be modded to boot from an sd card. Sounds great, that means you can make it play your own videos or even run x-windows! Well, maybe, but initially Emsoft failed to honor the GPL and would not release the source for their custom Linux operating system.
After a lawsuit was brought to have the software released[I forget the name of the group....it is a self-empowered group which helps get sourcecode released] - Emsoft did release the 99% of the code. They did not release: the driver for the displaying images on the LCD, the driver for accessing the SD card, and the driver for accessing cartridges. These 3 drivers are claimed to be written from scratch and therefore are not covered by the GPL. Without those 3 drivers, it is still possible to boot the device into your own custom linux version and get a command line on the screen - LCD displays generally have 2 ways to access, one is as a standard text screen and the other is for graphics. Text mode works, graphics is 'custom'. Despite lawsuits, these drivers have never been released. This is not an isolated case, I have a cabinet full of "open source" devices like the juicebox which I can't run more then basic text on. I have requests for source code for those drivers along with replies asking for large sums of money and an nda signature - all so I can use a little 2 inch keyfob digital picture frame as a mini monitor[requests I refused]
*Long story short:
=====
Legal precedent is that under the GPL the only source you are required to release is sourcecode which includes actually lines of code from the GPL licensed code. For compiled applications, all the code that is used to generate A SPECIFIC FILE must be available under the GPL. So two files:
gpllibrary.so and custom.so can be delivered in a single application and only the files that were used to generate gpllibrary.so must be released.
*Exposition
====
I'm talking here about the GPL as written...not the principles of the GNU organization[note, I did not say the principles of the GPL - the GPL is a piece of paper, it doesn't have any principles - while the license outlines a set of goals/principles underlying the terms - those goals are open to interpretation with different principled individuals having different opinions]. The GPL is used by many people who do not agree with all the principles of the GNU Organization - yet it's use by people is welcomed by the GNU Organization [at least, I am not aware of GNU ever asking Linus to stop using the GPL for Linux despite his refusal to upgrade to v3 where the legal terms more closely adhere to GNU's principles]
So, legally, the only benefit of LGPL over GPL is that you could use a tool such as HipHop from Facebook to convert PHP application to C code and then compile it into a single executable. Under the GPL, if you do that you must provide the sourcecode AND the customer is allowed to redistribute the sourcecode. I don't see that happening, but it sure would be fun!
The main difference between LGPL and GPL is not the legal terms, but those "principles". Many people assume that if software is licensed under the GPL, then the group releasing it has the same principles as the GNU Organisation - so out of respect one should follow those same principles in releasing your own programs. Whereas the LGPL is based on a compromise of the GNU principles[ie the GNU Organisation had to choose between their principles and not having to maintain multiple versions of their software in order to accommodate different libraries. They decided in the narrow case of libraries, they would compromise their principles.]
So it is understandable that many people who have same principles as the GNU Organisation would rather the code be under the GPL. It is just as understandable that those with different principles may want the LGPL. Neither group is "compromising" their principles - they have different principles. Well, I say neither group is compromising, but that is just the safe assumption to make. It is better to assume no one is compromising and that they disagree - then to assume and accuse others of being unprincipled. I
*Clarification
=====
*1: As far as I can tell, the principles of the GNU Organisation are not overtly concerned that a programmer provide source code to their customer. IBM has been doing that for half a century!*2 What they are concerned about is that you cannot keep your customer from creating something with your code and then giving it to someone else. IE once you give it away, those you give it too must also be allowed to give it away.
*Ancient History
=======
*2: As far back as 1970 IBM sold these things called "mainframes". When a customer bought a mainframe, they were given the sourcecode for the operating system! Before the GPL was even a glimmer of an idea, IBM was giving away sourcecode. Customers where free to modify the code for their needs and many did!*3 The catch though was that it was licensed so the customer could only use it ON an IBM mainframe and that the customer could not give it to anyone else. You could buy a copy of the software without a mainframe, but it was only marginally cheaper to buy the software than to buy a mainframe. This was to lock other companies out of selling mainframe hardware compatible with IBM's software - after all if it costs 10 million for an IBM mainframe, or 8 million for an X brand mainframe without software- as long as the software costs 4 million the IBM mainframe is the better deal.
*Amusing Anecdote
====
*3: My father worked for a while as a field service engineer for IBM. He would go out when a mainframe was malfunctioning and fix it. Some of his interesting jobs were for the U.S. Military. They would modify the source code for their needs, then a few months later something would stop working. Before he arrived, they would wipe the operating system and install the unmodified code. The changes to the code were classified. What it was trying to do was classified. How it was failing was classified. Running on a fresh copy of unmodified code everything was functioning. Fun times trying to track down which component, if any, was faulty.