All it needs is a medic & pyro or heavy or maybe demo... and all the sentries are gone. Of course you need an über and some kind of at least a bit teamplay orientated minds behind these classes... but that's mostly a public play issue ;)
Yeah, exactly - of course ANY obstacle may be overcome given proper teamplay BUT that is not a valid argument for our public servers - you can't change the way people play pub fights and so the only thing we can do is make the gameplay more fun and dynamic if there isn't proper teamplay taking place.
Basically the fact that good teamplay is required to overcome sentry encampments (yes, you can sticky them, but the demoman needs to be protected - otherwise he will be killed by other players) while setting them up doesn't require almost any teamplay effort at all makes e.g. Crovax' crusade to re-instate engineers on CTF maps quite pointless. His arguments are good (albeit repetitive ;-) but irrelevant for public matches.
Also, remember that SpA members always have an advantage over random pubies since we're using Mumble or in-game voice and that adds to the gameplay imbalance when we're always able to e.g. destroy sentries with good teamplay while the other team gets utterly destroyed by ours - even one or two SpA members more or less on one team can ruin the gameplay for everybody (except them ;-) and it's not because they are necessarily more skilled individually. This leads to all the recent rants about team balancing by e.g. Bucky etc. I think the way we started handling this problem is the way to go.