Many users work with modest-sized images, and so 24GB memory in a Mac Pro is a perfectly reasonable amount (and not very expensive at present in the context of a Mac Pro system or MPG Pro Workstation).
In my own use, Photoshop regularly chews up 10-15GB just for itself, for 24GB is not enough together with the other tasks I’m doing at the same time. So my standard memory configuration has been 48GB for a while, and I have found that it is enough for my everyday use.
Then there are those really big jobs, those billboard-size panoramas, gigapixel images, etc. With too little memory, the fastest Mac Pro slows to a crawl.
To be sure, 48GB is already a lot of memory, but the results show that going from 48GB to 96GB cuts the time dramatically.
It is the problem size versus the amount of memory that determines the benefits, or lack thereof. There is always a threshold at which more memory will help a great deal.
Had there been only 24GB in the Mac Pro for this test, the results would have been far more dramatic. Furthermore, my MPG Pro Workstation setup has an exceptionally fast scratch drive (triple SSD RAID-0 stripe), so that 48GB case actually would be a LOT worse on most setups.
Since a 4-core or 6-core Mac Pro can utilize a maximum of 48GB with Mac OS X Lion, that constraint is one consideration when choosing a Mac Pro model.
Other speedups are possible that are not memory related— such as the DisallowFlateCompressedPSD plugin for Photoshop 64-bit.