We recycle a lot of old cases in this household. With my screw-tapping hardware, we've even taken pre-1994 cases (non-ATX) and modded them to fit ATX motherboards. (The slots and slot-orientation of ISA and EISA expansion slots never changed when ATX boards and PCI were introduced. You just need to line up an ATX motherboard with the slots on the case-rear, mark and drill the ATX array of holes for motherboard-standoffs, and tap the threads in them (most likely 6-32 threads.)
At the same time, we upgraded two of my extended-family's computers from Pentium-4 socket-478 to LGA-775, choosing low-end C2D E21x0 processors in early 2008 and $70 mATX motherboards with onboard graphics. My family members mostly use office applications, e-mail and surf the web, or they play less-demanding games, so these hardware choices were appropriate -- saving them a few hundred bucks each when it came to paying me reimbursement.
[And imagine what we've saved on the purchase of computer-cases, too.]
Geez. Even a full-tower Gateway system I bought in 1995 came with an mATX "Zappa" motherboard for a Pentium-MMX 100-Mhz processor.
So I don't see what difference it makes, except for the reduced number of mATX expansion slots and some hardware-extras.