From what I understand, removing the offending define is required only for 64-bit builds and 64-bit architectures are already guaranteed to support SSE2 instructions. 32-bit builds can retain the flag and thus continue to disable SSE2 instructions and remain compatible with older computers.
Older CPU that don’t support SSE2 shouldn’t be excluded from coin generation in my opinion. They can still serve a vital role in block generation; I have many machines that don’t support the SSE2 enhancements, but still generate blocks all the time, even if they are only churning 300-400 khash/s