Michael Keaton was freakin' Beetlejuice the year before he was Batman.... and we're complaining that Evans was already a superhero?
Keaton was in only one non-comedic film before Batman, and Batman turned out ok. If we let resume prejudices completely control our casting choices, no one would ever be cast.
At this point, I'm not going to invest too much in arguing against the anti-Evans crew. When you think about it, every single comic book movie hero has its detractors. There will always be people who don't like Bale's Batman. There will be those who think Jackman is just too large to be Wolverine. Or those who think RDJ is too snarky to play Stark. Or even Ledger is not fit to be Joker.
I've just come to accept that no matter who is cast as Captain America, there will always be detractors.
It's not going to affect my opinion of Evans as Captain America. He's got the look, he's got the charisma, and I think he's talented enough to pull of all the demands of Cap's character. He'll need to show more restraint than in his other movies, but this is what he gets paid to do.
I'd rather Marvel choose an "already was" than a "never was."