I think Logan on the Avengers doesn't work because he's generally a loner who barely gets along with the X-men and because his personality should have him constantly at odds with guys like Cap, to the point where realistically they would probably throw him off the team. Remember, half the reason he was 'allowed' to stay with the X-men was because Charles wanted him there due to a sense of (for lack of better term) mutant solidarity, often over Cyclops' objections. There is no Xavier in the Avengers and most of the "big guns" there tend to be more like Cyke than Wolvie. I also think it overexposes the character to have him on two teams. But, obviously, current readers disagree.
Spidey is a bit more complicated an issue. Pre-marriage Spidey didn't want to join the Avengers because he considered himself a loner (there was probably also a bit of self esteem issue, namely, Spidey didn't have any). As he got older and matured it made a bit more sense that he join. However with the Mephisto reboot it is arguable that, again, he's more like the old Parker who wouldn't be a member of any group foolish enough to let him in.
In fact, I'd say the bottom line here is that Wolvie and Spidey both work best as "street level" heroes and, just as it is often difficult to realistically put Batman in JLA (either making him ineffectual or the "with enough prep time uber-Bat"), having Logan and Peter in Avengers is a creative challenge that too many writers aren't up to.