Is this still a viable means to help "balance" the classes?
Of course it is. The fact that it has never been done correctly/effectively before may tend to color some viewpoints. And I sieze strongly upon the word "help" in there. It is not a means to completely, utterly, absolutely balance everything about the differences in classes - but it
can HELP to do so.
AD&D screwed up the implementation of using xp progression to balance classes because it tried to do so by changing what you need to progress at 1st level to balance what you need at 18th, which is a completely whacky notion. Wizards are FAMOUSLY weak at 1st level - that means that as a balance they should need LESS xp to advance, not MORE. The most powerful class at 1st level should need the most xp, the least powerful the least. That symmetry should carry on through each level - as a character gains power relative to other PC's with the same xp total (NOTE: not level - xp total. We are talking about
asymmetrical advancement after all) the amount of xp he needs to advance increases, as a character class falls behind other classes it requires less xp to advance. THAT is using xp to balance classes.
XP simulates expertise gained through experience.
No. XP is indeed a meta-game measure for players. Gary made this quite clear in the 1E DMG where he explained that level advancement, by rights, SHOULD be achieved by spending time in a library reading musty tomes about magic or tilting at lists - NOT just by killing the odd monster and certainly not at all by tallying up the value of its loot. XP is firmly lodged in the "game" part of Roleplaying Game. Some have said simpler is better. This too, is not UNIVERSALLY true. Not everybody wants rule-free, rules-lite systems. Sometimes added mechanics makes it a better GAME.
Multiclassing is something of a seperate discussion all its own. Frankly, I agree with Lanefan (unable to xp again until I spread some around) and I would not weep if multiclassing were simply dropped altogether. What would or could it be replaced with? I can think of a couple of things but that's a different topic as I said.
The AD&D xp charts are difficult to analyze because they are logarithmic in progression and xp awards do not necessarily remain equal. Insofar as I've been able to, however, I find that they jump around a lot and do not meaningfully reflect ANYTHING that's actually happening with a given class. Combine that with the fact that neither Gary nor anyone else (to my knowledge) has ever come forward and attempted to explain just how an increase here or decrease there was actually going to accomplish something particular. My conclusion then is that when the AD&D charts were implemented there wasn't a point where those charts were intentionally designed to acheive given effects. They were just kind of thrown out there. The whole game was indeed still being made up as it went along. RPG game design was still in its infancy - it was still being DEVELOPED as a discipline. People have put WAY too much faith into elements of even more recent editions, much less the older ones. Why do you suppose that WotC does all this playtesting? It's because when it comes down to all these bright ideas they still don't KNOW if they're all that bright, if they'll even work. The ultimate way to figure it out is to PLAY it through and see what happens.
I also want to add again that this notion that classes actually CAN be balanced is hogwash. Even if you give two players IDENTICAL characters in every respect one of them is likely to do better than the other simply because the players differ in their abilities and dice results are RANDOM; at key moments a tip from one die face to another can have deep and lasting repurcussions. Not WILL - but CAN. It is a good thing to take steps to lessen those impacts but it is also a good thing to accept that we will never eliminate the inherent unfairness of dice and player skills. It is also a FAR more interesting game when characters ARE different in their capabilities, not merely clones of each other who simply use different
names for the same mechanical abilities.
It needs to be said as a reminder that D&D is a LEVEL-BASED SYSTEM. Attempts to eliminate any difference between characters of different levels is essentially trying to deny that. ACCEPT that D&D characters have levels and that they are never really going to be equal and you'll have a lot fewer problems. The more you try to divest it from this essential element the less it will feel like D&D - the less it will BE like D&D. At some point rather than trying to bend D&D to be what it ISN'T you'll do better to simply go play a different game that IS what you're after.
Just look at all the ways people handle xp and xp progression - including ignoring it entirely. There isn't a "one size fits all" solution here, and in fact more than one answer can be the correct answer. There are even possible approaches that haven't been mentioned yet. I see the advantages and disadvantages to both the AD&D varied charts per class and the 3E universal advancement. I won't be able to complain too loudly about either one and might actually enjoy an entirely new approach.