> Windows requires SSE these days, since Vista IIRC, so the problem 
> is probably someone on XP.

Windows 8 is the first to require SSE2 [1][2]. I'm sure it's no coincidence that this became the default in VS 2012. 

There is probably a small minority of users that upgraded to 32-bit Windows 7 on old hardware. Python 3.5 excludes them, but I don't think the build should switch to using /arch:IA32 just to support them. If a user on such a system really needs 3.5, it's possible to create a private build without SSE2. Most packages that have extension modules will also have to be built from source instead of using prebuilt wheels.

[1]: http://windows.microsoft.com/en-us/windows7/products/system-requirements
[2]: http://windows.microsoft.com/en-US/windows-8/system-requirements