I've just noted that on Stack Overflow, .Net users seem to have created a load of tags like "2.0", "3.5", "4.0". This seems to me to be a bad idea, (though I realise that some .Net programmers believe that SO was created for them alone) and they should all be retagged as .Net2.0 etc. and the creation of numeric tags be banned in future.
A quick look through the questions tagged 2.0 reveals that it's not just .NET users. That's unfortunate, since it will make retagging a manual process. That ambiguity makes me even more strongly in favor of banning the tags.
As Bill said, the questions would have to be manually separated in most cases.
I created a Data Explorer query to find all the numeric-only tags in the system and the question count for each.
It doesn't look that bad to clean up. Basically go through each one, retag the ones that don't belong to a major group (i.e., .NET), and then get a mod to merge the rest. There aren't very many popular groups in the list, which is nice.
I think each tag can be put into a different tag that's not numeric only, i.e.
Just a small notice. This is going to break stuff on [Drupal Answers]. They use numeric-only tags for versions of the software, and this is perfectly fine because the entire site is about Drupal.
It is also possible that a similar case exists on another site.
Implied from that, I'm against this proposal.