1)  This will slow down some plug-ins (Adober reader, for certain)
     which rely on being able to get to the data as a file; if we can't
     store it on disk in our cache, we'll have to stream it to a temp
     file instead.
2)  The size of the memory cache is capped at a much lower number than
     the size of the disk cache; it's a lot easier to fill it up and
     start evicting things.
3)  The disk cache, unlike the memory cache, persists across restarts.
     This means that you trade off slower startup (due to none of your
     restored tabs being in cache) and slower initial visits to all
     sites for slightly faster repeat visits to sites, as long as you
     didn't visit too much stuff in between.
4)  We have ongoing work for Fx4 to make use of the memory cache as
     well as the disc cache for the same content (right now we use one
     or the other but not both for any given resource).  Once we do
     that, you'll get all the benefits of this "tweak" without the
     drawbacks... and using the tweak at that point will be a strict
     loss.

-Boris