Because just like every FPS has trophies for completing certain things online they usual have some sort of content you can only get online as well maybe they wont but i'd be suprised.
When you say "trophies," are you referring to achievements? MW2, one of if not the most popular online FPS for consoles, has all single player achievements.
You may not need in game support but it makes things easier for clans that do not have the time to go onto MLG and setup matches and make account and track everything through there.
Well in my eyes,that clan really isn't that serious, are they? If you are "serious" about a clan, and have a mentality like "I'm willing to work hard, but not that hard," then your clan is nothing more than a bunch of guys that like to play the same game(s). Any clan that is serious about compettion will register with a league.
Not to mention you cant argue clan support extends the life of a game because after leveling up, getting all ablilities, all trophies ect the game can get a little stail having something to work towards and another level to play on keeps alot of people playing longer instead of moving on to the next game ensuring they are around for DLC to be purchased.
This can be argued all day. You know what else extends the life of a game? The ability to create 16 different characters, each with their own customization and abilities. Oh, and bot matches help a lot too.
And just because you play a support class does not mean you are a team oriented player alot of people play as medics jsut to get exp from reviving and healing other players simple as that.
You're right - a lot of people do. I don't. Remember, I was playing competition level matches on PC as the team's primary Medic. And this was in games that didn't even have XP.