Hi Nickolas,
Yea, there is nothing that we could not do with Joomla! core architecture. (99%, but not 100%)
But the important fact in some area is the TIME. When you have a little time (due to customer's budget or any other reasons), going on developing extensions need much more efforts. On the other hand, using ready extension from JED is not a case when you could not trust to other developer's qualifications of code. (all of extension developers are not as good as you in programming)
In the previous releases of Joomla! only updated files are provided in update package, but nowadays the pattern is reversed.