Blender is, for the most part, cross-platform in nature. Surely, there will be some platform-dependent issues that might be relevant, especially for Unix/Linux users. However, most 'platform-related' questions are probably going to be outside of the actual Blender development process. For example, questions about image export, Blender's compatibility with other, OS-specific software, and external image manipulation software (IE GIMP and Photoshop). I feel these tags are going to have an abnormally high percentage of off-topic posts, so should we keep or ditch the platform tags?
Whether something is off-topic or not will be filtered naturally through voting habits. I suppose we will see soon enough how many degrees of separation will be considered appropriate by the community.
There will be instances where it is useful to include a platform tag, because platforms can have different behaviours. Think of keyboard shortcuts, path variables (for python related things), and graphics drivers.
I agree with @Leathe above.. as long as the majority of the problem is with Blender it is valid. Depending on how it is worded and what the OP seeks to achieve however, is what will be the deciding factor won whether it gets an answer etc.
I think that they will not be so many. Also, as the site is Blender specific and Blender is multiplataform is normal that you need to specify you system in the answer (if related).
For example I want to ask how to change Transform Orientation with keyboard because in Ubuntu (and many others) Alt + Space are a system combination to show app menu. I think this question is ok (although is my own question, so it may be a little biased viewpoint).