Actually consider that Mordecai isn't just a sniper, he's more or less the game's "thief" character as well. The abillities on his Bloodwing allow it to increase drops and quality of drops, meaning that it's not just an attack but also basically an improve loot-magnet mechanic that exists to ensure that Modecai has more money/items/better gear than other characters at any given point in the campaign assuming he builds it up.
This is also a big part of why he would be welcome in groups, since he increases the loot produced. Defined that way his bird seems a little less cheezy, even if it isn't as lethal.
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Also:
I'm not sure if I'd consider Borderlands a "Rich Gameworld" it's pretty shallow, which isn't a big deal for the kind of game it is, but still I'm surprised to hear it praised on that level. I was hoping a sequel would actually round out the world because while it has the potential to be interesting, to me it seemed to lack substance.
For RL reasons this is another game I played shallowly, didn't get too far with, and mean to go back to (Thanks WoW!) but I *HAVE* played it, and while some of the NPCs are amusing there really isn't that much backround color, lore, or anything else. The game sort of reminds me sandbox sized Team Fortress 2 map. It seems like there should be a lot there, and your waiting for somene in the actual game to fill in the details... but really there isn't anything.
The world could have benefiteed from more RPG, some extra NPCs to have backround conversations and such and fill in little bits indirectly, more flavor characters, etc...