Bob, Hawkeye isn't just "really skilled with a bow and arrow", he's also a mutant with incredible eyesight similar to that of a Hawk if I recall (hence the name) letting him see things miles away. On top of this he's a gadgeteer who creates arrows for that bow of his that pretty much do anything the plot demands. He needs an arrow that's non lethal, short circuits robots, freezes someone in a block of ice, lets out a massive sonic burst to hurt the ears or whatever, this guy pretty much has it. He's pretty much the quintessential "power pool" character from certain Super Hero RPGs, because like Batman's Utility belt, despite pretensions of realism, this guy by definition always has exactly the right gizmo the plot requires.
Arguably Hawkeye is probably on a similar level to Iron Man, maybe even worse. Heck I seem to remember that he's even taken Iron Man down in the past when various plot lines have required it by using arrows that basically fry his armor with focused EMP (which is a trick The Mandarin uses all the time... certain guys can put out enough force to bypass his shielding).
Also remember that this is a guy who due to you know having super vision, and gizmoteering, is basically able to castrate a gnat from like two miles away.
Hawkeye *IS* pretty much a Marvel Comics "also ran" who rounds out teams, but he's been in the comics alomost constnatly because he's pretty popular, where tons of other characters have pretty much faded into total obscurity by being unable to even hold down enough interest for a team spot. He's also had some AWESOME moments over the years as I pointed out, which is part of it.
Hawkeye definatly fits in with The Avengers usually, and despite all impressions, and I think one of his strengths is that he's overlooked. He's not a wimp by any stretch of the imagination.
Oh and the guy also has some pretty impressive skills as well, sure, the guy did take down Deathbird (evil empress of the Shi'ar) one on one back when The Avengers were out of comission and he was doing solo work. He pretty much walked through super-security to apply for a security job right before that because you know... he's a Gizmoteer, and doing things involving super tech is definatly part of his skill set.
I'm ranting about a minor point here, but I think he's being dismissed easily. What's more he's one of those characters who while not a founder has been on The Avengers active roster pretty much forever. If your going to bring Captain America in due to the amount of time he's spent on the team as a central character, then it's only fair you bring Hawkeye in as a supporting role because he's one of the defining supporting roles of the whole crew.