Well, please note that the AVX family relaxes the alignment requirements and with the newer ones one can even completely ignore alignment provisions.
(For the past 10-11 years, Marco). FPC has good AVX (+) support. Use SSE2 and its alignment requirement only if you need to support legacy hardware, I guess.
One reason is SSE2 being part of the X86_64 standard, and keeping the stack aligned is cheaper than aligning before every (SSE2) operation.