I think it would. At least the Class-and-a-Half rules, I would think, would be worth it. I don't have experience with "full on" multi-classing from the PHB.
Consider a group whose members average around 60,000 xp. Most characters are going to be 6th or 7th level. With the same XP total (approx. 60,000 xp), you could have a Fighter 6/Wizard 3. Or a Barbarian 7/Rogue 3. At 100k xp, most single classed characters will be 7th level (with a few 8th). The aforementioned Fighter/Wizard will still be 6/3 (and will be so till about 110k, when he moves to 7/3). The Barbarian/Rogue, however, will have advanced to 7/3 at 74k (or thereabout) and will move to 8/4 at 130k xp. Some single classed characters would still be 7th level! Now, granted, some combinations start to slow above this level, but I still think they have the ability to keep pace with single classed characters.
So, from an XP standpoint I think they'd do fine. They're not too terribly lower in level as to not be able to contribute to an encounter. Just be mindful of the restrictions, so you know which class should be the principle and which one shoudl be the supporting class.
Far be it from me to disagree with Ronin, but I think unifying the XP chart is a terrible idea. No offense, sir!

The mechanic was okay in 3.x but it was flawed, as some classes were clearly more powerful than others. To me, the different XP charts were a welcome sight once more, as it threw the illusion of balance out the window and acknowledged that some classes had advantages over others. And the different XP requirements were used, one again, as a balancing factor, or sorts. It gave the classes without the heavy firepower a little more "oomph" by giving them lessened XP requirements for levelling. It also allows for tailoring to the specific class by making a particular level a little longer to get to when they are about to get a powerful ability (such as teh level before the fighter gets his second attack or the monk gets his death attack). I'm not a stickler for balance, either, which is why I love the non-unified XP charts - not everyone class was (attempted to be

) balanced against one another. But to each their own.
