Whilst I do think that topics in chat are a good idea (ignoring the time zone problems), I for example, have no interest whatsoever in live bearers. I also suspect that most of the regulars wouldn't either. Therefore, you would have a bunch of regulars who would be bored out of their head for the hour or so that the particular topic was discussed.
Much better if, as Andy says, the people that come into chat on a more ad-hoc basis actually stay and give the people in there already a chance to say hello. I often join when I get home from work yet might not be active for half hour or so.
So, if you join and there are people there actually say hello and join in rather than running away again. If you want to talk about something in particular then start talking about it.