AbdulAlhazred
Legend
I'm aware of the danger, here, but I'm not really convinced by it - especially not in this example. The Fighter gets most of its benefit from marking when the marked opponents are adjacent to the fighter - Combat Challenge (combined with Combat Superiority - the combination is a real hard lock!) is generally far more of a control than the -2 to hit from a mark. That generally means that Area powers are not that big a deal. Close powers are very useful for the fighter - but they get several of those already. Better yet, they get Close powers that pull enemies towards them, rather than push them away*.
A more problematic issue might be a fighter getting Wizard control powers that make damaging zones that the fighter can move enemies into, but given the action economy I'm not even sure that that is too big a deal.
Overall, for most non-At-Will powers, I'm not convinced that there is really a problem; the MC character is paying at least a feat for the ability to swap anyway (so, arguably, it should/could be a teensy bit better than a power from their own class). Individual powers might cause an issue (I haven't checked through them all), but as a general rule I don't see a problem.
*: I should say that I have a PC Dragonborn Fighter I run for; they get dragon breath Close Burst that marks every target. We have been playing from level 1 to early Epic, so far, and it's really not a big issue.
I think it was a more radical concept in the PHB1 days when you had 8 classes that had NO real way to get each other's powers except the MC feat chain. There were a lot less powers back then, and giving your fighter some sort of area burst at range 10 power doesn't seem bad. Nowadays its just not THAT hard to get a power from another class (leaving hybrids as a bit different question). So, yeah, if you have to take 2 feats and then you can just swap by retraining and get many of the other class' powers its not like the character couldn't have gotten the one or two really most useful powers anyway (and how many people will take more than a small number of the other class' powers probably anyway, unless its a really poor class like Seeker with crap powers).
The at-will seems to be the key thing to reserve for PMCing. That's sort of the cream and it makes a pretty good incentive to PMC. There definitely need to be other ones, especially since you're basically taking away the rest of what PMC got, power swaps.
The other alternative of course is to add a couple more Paragon feats and just forget about PMCs entirely. If you take those feats you can get access to the other things your 2nd class does, and the at-will, and you're basically all set, you can still take a PP. "Paragon Multi Class" can be the generic feat that gives you the at-will.