AFAIK, MMX was more for fixed point math (as also pointed out by another commenter) which would explain why it wasn't used much for 3D. You tended to see it more in image and sound processing code. Maybe the reviewers got it mixed up with 3DNow.
replyMMX did help significantly with software 3D rendering. I worked on a software rasterizer that benefited significantly from it. But it didn't take long before even a well-optimized software renderer on a high end CPU couldn't keep up with a low end GPU on a low end CPU.
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