If properly implemented it would mute would have an effect on server load. Also from my understand, which can be wrong, LFS is P2P based. Sure there is some communication with the server for things like chat messages, race restart etc - but the main physics stuff is done P2P which means the server really isn't loaded that much to begin with; although like I said I could be wrong.
Your point is still valid VoIP would increase bandwidth. Just because of the 1 programming developer doesn't mean that once physics are top notched that this type of feature can't be added, so IMO that is an invalid argument, although I agree this shouldn't be high on the priority list.
Rants/Muting is a controversial topic. Of course you will have the idiots in the game fooling around for no good reason, so mute is necessary - in any VoIP system, so that isn't making this special. That said, a team radio is a realistic thing to have, including between two cars (on the same team). Sure, chit-chat is saved until after the race by almost all drivers; but there does need to draw a line between sim vs game. I am all for 100% realism in the driving department, but considering an online community, race restarts every ~5 minutes on casual servers and a few other limitations I think that chit-chat with others is acceptable and making up for other things. In a league event however I think the option should be able to turn it off completely; or even force administration team to be heard; (with non-broadcasting available so the admins can talk to driver a without interrupting the rest.)
Yea, wall of text - sorry.