As a semi-regular I don't understand the need for mumble. The in-game TF2 voice works great and everyone has it by default.
Firstly we dont just use mumble when we are playing, there are a lot of us that are on it when we are not playing. even when we are playing there is a lot of general chat/banter, if this was said in-game it would be bad for a few reasons
1 it splits up the existing community (exactly the opposite of what we want) as most of the time you would only be able to talk to those members that happened to be on the same team (and putting alltalk on all the time prevents tactical team chat)
2 it would clog it up with non relevant chat that some players would not want to hear
3 it could prevent others from being able to pass on useful team specific info eg warning of a spy's location
From a technical point of view, mumble has a better audio quality than in-game chat, also I'm not sure what the latency difference is between the two as I have not tested this.
Having said all that I am guilty of sometimes using mumble for tactical chat when I should really say it in game, its something for me to work on. Secondly please come join us on mumble, you will have a much better overall game experience. If you need a hand setting it up, read the guide posted elsewhere on the forum, post your questions/problems or join the ne irc channel and ask there.