Zen Toombs said:
For the most part, XP is shared so there isn't really a thing called "killstealing". Work as a team, even with randoms.
The issue is that even if
you don't care about your own score,
other people, more competitive people, do. The scores exist, in part to see how much you're doing in relation to the rest of your team. If the vanguard (or, as many people have pointed out, infiltrator) is 20-30k above everyone else by end game, that means that they got the brunt of the game's kills.
If you're okay with that, great! More power to you. But I bet the rest of your team would have liked to get a few of those points, too.
Someone else mentioned that they go AFK when a human vanguard is in play, and that serves to show another point, that a vanguard doing everything is boring for the rest of the team.
-snip-
Lastly, to respond to the "it's a team game" portion of the quote. I already covered this on page one as well, but I'll be more than happy to reiterate: If the vanguard is doing everything, zipping around the map quicker than any class, killing everything before the rest of the group has a chance, it stops being a team game. Generally, you are not working as a team when you charge+nova your way across the map.