This change is mainly to benefit 32-bit systems where SSE support can't be assumed by default, but with the i965 driver, more often than not it can be assumed an Intel Core 2 processor or newer is in use. (The older Intel processors are generally using the i915 driver.) By setting the -march=core2 flag, for i386 builds SSE would now be used for floating-point math and cmov instructions, plus other performance optimizations.
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This patch was ultimately rejected since it turns out there's still some old Pentium 4s that could be found in an i965 driver configuration where things might break.
Then why not use something like -march=i686 -msse -msse2? That would enable gcc to use cmov and sse/sse2 instructions and the binaries would still run on a P4.