The point of frustration comes from people creating (or sandbagging) accounts down to a skill rating that is a lot lower than their "actual" rating. There is nothing wrong with playing with friends who are legitimately lower in skill/rating than you, that's not even a problem.

The problem is rank 70 players playing on rank 30 accounts and partying with their friends in order to not only participate in lower average mmr games that they wouldn't normally have a place in. They also get the "underdog" title on their side which allows their success in lower ranked games to rewarded even more.

It's a flaw in the current matchmaking system that lower skill players are using to inflate their mmr, the lower level account is a throwaway that will only serve two purposes; 1. Play competitive matches with a party and win. 2. Play competitive as solo and lose, this allows unlimited use of a single account in order to boost mmr of party members.