On the one hand, there are some OS-specific questions that arise from time to time. So, having OS tags allow one to sift through them conveniently. On the other hand, Mathematica is designed to be cross-platform, and such issues appear very rarely (mostly, I think, about window decorations and keyboard shortcuts). Also, Mathematica users often try to write portable, and most questions involving OS-depend details actually give details for all three supported platforms.
I think, these tags are useful, for at least two reasons:
- Things like installation, administration, short-cuts, and also some Mathematica functionality are OS-specific. And people will appreciate separate tags, so that they can search directly, say for MacOSX, etc
- Even if the major part of Mathematica itself is cross-platform, many other things to which it can connect, and with which it can inter-operate, either are not, or have platform-specific features.