Yes, some congressional districts would become more important under the district system (swing districts in non swing states) but just as many would become less so (non swing districts in swing states).
Why would a candidate spend a lot of time in safe districts anyway? I don't think there's any sensible system that would make it worthwhile for a candidate to focus a lot of resources on the Bronx or the Oklahoma panhandle.
Why is it any less sensible to attempt to get these votes than the votes of anyone else?
Well, for one thing, those voters aren't sensible to begin with. McCain could have spent the whole campaign in D.C. and still not broken out of the single digits. Obama could have spent the whole campaign in Oklahoma and still not have broken 40%.
The candidates should focus on undecided voters who are willing to become informed on the choices they face, not on party-line hacks.
Ah, so the mask is off, I see. You are wise enough to know which voters are more important than others and whose votes should decide the outcomes of elections.
Thanks for educating us rubes who are naive enough to believe that all men are created equal and should be treated as such.