One thing that has occurred to me while looking at the multiclassing rules--they really *do* give a lot of access to the other class. Let's take a look at a level 20 character who has gone the "multiclass instead of paragon" route:
According to page 209, the multiclass feats are:
0) The multiclass feat for the class in question.
1) Novice Power (swap one per-encounter attack power)
2) Acolyte Power (swap one utility power)
3) Adept Power (swap one daily attack power)
The paragon multiclassing choice further gives you:
11th level: swap one at-will power
11th level: 7th level encounter power
12th level: 10th level utility power
20th level: 19th level daily power
At 20th level you normally have your two (three for humans) at-will powers, four encounter powers, four daily powers, and five utility powers. Those powers are expected to be of level:
At-Will: 1, 1, (1*)
Per Encounter: P, 17, 13, 7
Per Day: P, 19, 15, 9
Utility: P, 16, 10, 6, 2
* If human
With multi-classing (assuming you always keep your second-best slot in your secondary class, and keep the best one for your primary class), we have:
At-Will: 1, 1M, (1*)
Per Encounter: 17, 13M, 7, 7M
Per Day: 19, 19M, 15M, 9
Utility: 16, 10M, 10M, 6, 2
If we assume that the regular "replace one encounter" and "replace one daily" features of leveling up can be applied to the paragon multiclass powers, then at level 30 you'd expect something like:
At-will: 1, 1M, (1*)
Per Encounter: 27, 23M, 17, 13M
Per Day: 29, 25M, 19, 19M
Utility: E, 22, 16, 10M, 10M, 6, 2
Note that one of the multiclass utility powers can be shifted around among all of the slots, whereas the other one is limited to being only at 10th level. Likewise, the high-level multiclass per-encounter and per-day can be shifted up or down as you desire.
What am I trying to point out here? Well, this: If you take the paragon multiclassing path, fully half of your at-will, per encounter, and per day attack powers are from your second class. Yes, sure, it's "only two powers from the other class", but you only get four powers total!
Utility powers is the place where this breaks down, and I might be tempted to allow one of the level 2 or level 6 utility to be swapped between classes for free at high levels.
In short: there's no good reason to allow more than one swap without the paragon multiclass approach. If you're going to be swapping that much stuff from your secondary class, you should multiclass, or you should have picked a different primary class. If you take a paragon class from your multiclass, it's the same thing, basically: Half of your powers are from your base class, half are from your paragon path. If you wanted more than that, you should have used your "other" class as the base class.