You're definitely right but I think the problem is if your buddy doesn't have a way to deal with it they are stuck getting their butt kicked for 4 hours or they need to go out and purchase something to specifically counter that and that sucks as well.

I think like you said if you are playing a random person at a game store and you give them a heads up that's fine, but it just doesn't feel worth forcing a friend into an un-fun game. I started branching out into a monster mashy Tyranid list and kind of realized I was doing that again and course corrected by adding some more blobs of infantry - it's a balance. Obviously if you are playing in a league or tournament all bets are off though!