Nearly any tag that would be appropriate for a site-wide chat room would be far, far too broad and useless as an actual tag for questions on the site. We should not compromise the integrity of the main-site sorting system for the sake of the chat; the chat system is a tertiary function in the Stack ethos and should bow before mainsite and meta in all things.
This means that we either have no tags on the chat, or primarily made-up ones. Many sites go for the former, and that's fine--but kinda boring. Some sites find a particular use for chat tags: RPG.SE chat has tags specifically designed to clue in newcomers that it's not a chat room about explosives, video games, or legacy programming.
Since we don't seem to yet have a particular purpose the tags need to fulfil, we're having some fun with them. They can get changed at any time (another reason not to try backing them up with main-site wikis purely for the sake of an impermanent chat conceit), and no doubt will as the chat culture grows and changes.
So in order:
- Chat tags aren't necessary, but they can be useful and/or fun! I like 'em.
- It's unlikely existing main-site tags will be useful as chat tags, and we shouldn't invent main-site tags just to accommodate chat.
- If people are confused, they can ask. A lot of people don't even notice chat tags at all, in my experience. Tags shouldn't be misleading or offensive, obviously, but otherwise I'm seeing this as trying to fix a problem we don't yet have.
- Tags will not attract chatters to generic site rooms. In my experience people either go straight for the room they already know they want, or just click on the most active rooms in a kind of "go lucky number 52" gamble and see what they get.