This is sufficiently subjective that I can't really argue except to say "I disagree". The relative power of class features and powers is very variable. I don't think it's safe to say that someone who takes paragon multiclassing is guaranteed to be weaker than someone who doesn't, and I can think of a number of builds that require paragon multiclassing to be possible.
I do think it's safe to say that the quality of paragon paths is highly variable, and that some are just so good that they overshadow any other choice when it comes to achieving certain goals. Others are pretty bad.
As an example: If presented with a choice between doomsayer and wizard paragon multiclassing, I'd say it's a tough decision. Doomsayer gives some average-powered dailies and encounter powers, plus some class abilities that are pretty bad (because there are almost no fear based powers). Multiclass wizard would let me swap out eldritch blast for an at-will aoe minion killer and some powerful controller-type spells. AOE damage means I can rack up those bonuses to hit before I unleash my dailies.
All up I think paragon multiclassing is in a good place. I think some of the paragon paths just need tuned a bit.