Don't worry about the Harry Potter reference, Captain is just calling us Slytherin because we beat them and our insults are better
Truthfully all the alliances have skilled players who have had champions and number 1 fighters in their camps to help you out.
Alliance rosters under reference will tell you what players and camps are part of each alliance -
http://www.mmarmy.com/forum/ubbthreads.php?ubb=showflat&Number=362592#Post362592Alliance competition event scores are totalled for a prize to the winner at year end, and each group runs their own internal events and discussions as well.
An alliance does add a lot to the experience of the game for most. You get in on more discussions of strategy and events and kind of gives you a team to play for and root for. All alliances have some clothing in game as well.