I'd say this is true for many games where performance is not a terrible issue, but I wouldn't say "most" games. Lots of libraries do the SIMD optimizations for us and performance would probably be halved if they didn't. And in the game code it only takes a few days to spot the bottlenecks and optimize them. Any worthy game programmmer knows a bit of assembly.
But what the poster probably meant is, is it useful to have a dual processor with SSE2 support for gaming? My answer is no. If the game doesn't run on a single processor, it's not a good game. SSE2 is not very useful for games because it processes two double-precision floating-point numbers in parallel. SSE on the other hand processes four single-precision floating-point numbers in parallel. Since double precision is nearly never needed in games, only SSE makes a difference.