Since Joomla 1.6 and 1.7 were bridges to 2.5 which is the long term release, I was wondering if there was a reason to keep all three. Especially since any extension for 1.6 should work with 1.7 and 2.5 and no one should be on 1.6 or 1.7 anymore.
Generally, version-specific tags are discouraged. Most people end up using them when they don't apply, i.e., on general questions where the specific version in use is irrelevant.
Really, probably 90% of questions should just have a joomla tag, no version necessary.
That said, these types of retags are not something that can really be carried out automatically through the creation of a synonym. People who did use the version-specific tags correctly should not be forced into using a generic tag just because others don't understand how the tag system works. Someone who is knowledgeable about the framework should really go through the questions and retag them manually.
Once that's done, then we can look into creating synonyms, but it's probably unnecessary. Joomla 1.6, 1.7, and 2.5 are different versions, and I imagine that the differences are probably at least somewhat relevant, else they wouldn't have incremented the version numbers. Whether those different versions actually convey any useful information with regards to particular questions is a different issue, but one that has to be handled by a knowledgeable human, rather than a computer or script.