Should these two tags be unified? I don't think there is the need to have separate tags for support's structure and material, since the material is in most cases the same as printed. In the few cases it isn't, it would still be fine to have it in a unified support tag, imho.
I have to disagree with this request because the two are technically different (and I mean technically in literal form).
- Support material helps define relevance to a specific material type, ie PLA/ABS, PVA, that lime stuff that MakerBot tried selling, wax, etc.
- Support structures helps define relevance to the existence of support structure being generated and/or their physical dimensions.
So, I guess you could say that Support Material is a what question (as-in what is it made of) and Support Structure is a how and why question (as-in how is it made and why is it needed).
I'd also like to point out that you said
I don't think there is the need to have separate tags for support's structure and material, since the material is in most cases the same as printed.
A large factor in maintaining the quality in a site like SE is adhering to all cases, even if that means having a large number of tags to keep track of.