I think on a typical day, Tulsa doesn't have enough traffic to warrant use of tac channels for every incident. You might have 4 or 5 engines out on 4 different traffic accidents/medical runs, etc. and all can effectively "share" dispatch, even with dispatch. Tulsa just isn't that busy.
On the other hand, I have heard a few times when a working fire STAYED on the main talk group, and that always creates problems, because, while two working fires at once is rare, two or three medicals in other parts of the city, while a fire is ongoing is not uncommon.
The real chaos is those rare occasions when there are two working fires at once.
Think about it - how many times do you hear "E-4, breathing problems at the Day Center - 415 W. Archer" and 3 minutes later you hear "E-4 on the scene, being waved off by EMSA, we're in service". It takes more time ON the dispatch talkgroup to get them to go "off" the dispatch talkgroup than just to make their call. Even when they beat EMSA (often) or are needed to help, there's still little traffic unless it's a car wreck on the interstate and they either can't find it or they want the second unit to take an alternate route or to block traffic somewhere.
The current policy of "fireground tac talkgroups for fires" makes sense, at least until T-town gets a busier. And when that happens, the solution (in the works already) is a bigger dispatch center with more consoles and more dispatchers - THEN you can go to tac groups all the time without confusing the dispatchers or "losing" traffic.
As far as the "regroup" goes, as I understand it, there's a problem both ways. Manual switch, someone always has an HT turned off, because it feeds back with the truck radio while enroute, then when they get on scene and turn it on, it's still on dispatch's talkgroup. Auto-regroup lets the dispatcher "force" (at the battalion chief's direction) all the radios to "switch" automatically, but then they are on that talkgroup only. That causes problems at the really big incidents where they use two tacs at once. I've heard that 4 times recently. Staging had it's own group during the St. John's parking structure fire, the Airgas explosion, and two Hazmat incidents where Hazmat (and a couple of support units) were on "H" while the perimeter/decon/water supply, etc. were on a Tac.