Due to how buggy or weird some of the achievements are, and that sometimes you almost need to run private games to get them without screwing up random peoples games (eg buffing achievements, some of the map ones) it would leave lower leavl players without friends to help them at an even bigger disadvantage if it gave you a statistical boost.
Teamstacking: Sometimes friends want to play together, sometimes as Helmic says, the losing teams range quits and is just filled in by noobs, etc. Also being a higher level means lots of people just don't want to fly VS you, and if you are say an experienced engineer it can be hell playing with a pilot who knows jack all and wont listen to advice.
Thats another thing, some days you just don't want to have to micro your crew, but just play the sodding game :/ </end OT rant> (PS QUICKJOIN IS BAD AND YOU NEVER NEED MORE THAN ONE GUNNER)