Yeah there are pro's and con's to being 74, and some people do it just for lulz.
First of, they don't need as much hit (if some at all) as you normally would and other players would need quite a bit more hit to be sure to negate all them baddy misses
Paladins get wings at 72 (and upranked at 74) and as ret, that's a major boost - this allows them to cast Hammer of Wrath every cooldown. HoW being the paladin execute and you can easily crit 10k+ on a fully res-gemmed healer. Every 6 seconds!
Druids get Cyclone at 74 which is another major boost as cyclone is a massive booster for a CC chain.
Mages get 2/2 improved counterspell regardless of specc.
And there are plenty of other examples, some being more useful than others.
So you might ask yourself: Trololol, why don't everyone level to 74?
Simple - as you may know, the higher level you get, the worse scaling of your rating. You'll lose severe amounts of crit, haste and ultimately resilience percentages which makes 74's a lot more vulnerable to heavy burst than players at 70.
Sure, they might have slightly higher HP and slightly higher SP, but they take a lot more damage than they usually would.
This is why they're stacked on damage gems such as agi, str og int, as they're simply sacrificing too much for damage for too little resilience percentage if they stacked otherwise.
And yes, you have to join and preferably be the leader of a XvX team and THEN level up. But you're forced to stay in the team regardless of stats.
Have fun