I would first like to say that I am proud to have a grand total of 0 rejections in matchmaking, though I have had previous experience using it back when I didn't have an xbox and only played at my friend's house.
I personally dislike it because if playing with a full party against randoms isn't unfair enough, armor lock makes it enough. Often, you see people in the same party (or randoms that cooperate well) double teaming with armor lock, one person going into armor lock when they are about to die while the other finishes the guy off. To me, this is entirely unfair to the solo guy, who will only win if the teamers suck, he/she has extraordinary skill/experience, or his/her teammates come to bail him/her out. Other times, when you break the opponent's shields, he/she goes into armor lock and forces you to wait/leave, or his/her teammates come to bail him/her out. Finally, if dodging rockets isn't enough, armor lock allows players to easily kill opponents using the sword, which is already possible using a more skilled method, parrying. While I'm not for sword noobs, but I don't think this is fair method (my recommendation is that the armor lock emp is absorbed by the sword, effectively negating it) either way.
In my opinion (though I'm sure that many will disagree), is that armor lock should only allowed in playlists with vehicles, as armor lock can be effectively/honorably be used to stop vehicle noobs.