I think alot of the benefit of a conventional repeater/simplex system is being missed here. Let's say a FD uses a conventional VHF repeater as it's fireground channel. The repeater-out is 154.415. The repeater in is 158.950. Units get to the fireground and, per SOP, are using the repeater. A crew gets inside and discovers that it can't hit the repeater anymore. That unit switches over to the talk-around version of the fireground channel (154.415 simplex) and goes right on working. Chances are, that interior unit can still HEAR the repeater, so they'll still hear everything going on. And units outside who are still on the repeater channel can HEAR them. A well-established department might even have a mobile radio in a chief's vehicle programmed in reverse (so that it receives the repeater-in frequency and transmits on the repeater-out frequency). This way, they could hear and talk to a unit on the repeater channel who cannot connect to the repeater.
Sure, there are options for simplex operations for departments using 800mhz trunked radio systems (or any band TRS) but there is no continuity like I mention above. If your radio is set to the TRS, and you go inside a building where your portable radio cannot connect to the TRS, you have NOTHING. You could switch to the simplex channel in your radio and hope that someone else is listening. But you have no way of knowing if anyone will hear you.
I strongly believe that fireground operations should utilize conventional/simplex radio systems. Dispatch-to-command is fine on a TRS, but you just can't be 100% sure about on-scene communications. Also, while 800mhz is wonderful, VHF has a strong place in the fire service. It's got the power to get things done. I'd even take UHF over 800mhz. Just my 2 cents.