I like the unified level advancement table.
Levels are basically just a measure of character ability. If you don't have levels, you'd have to fall back on another method. GURPS has Character Points. In the D&D system prior to 3e, the traditional alternative was XP. Presumably, characters with the same amount of XP were roughly at the same level of ability, although it was also possible that the game designers did not deliberately set out to "balance" the classes in this way. In any case, even if they did, the multi-classing and dual-classing rules played havoc with the system.
But why would you need a system to measure character ability in the first place? Well, you don't ... if you're an experienced DM. If you are an experienced DM, you will be able to set encounters for your players that are challenging but not insurmountable. You will be able to tailor challenges for each member of the party so that they feel useful, even if they are at different levels of ability. However, if you are not, you might have a walkover, a TPK or a disgruntled player on your hands.
If you're not an experienced DM, it is probably better to have some kind of "training wheels" while you earn your DM spurs. The 3e philosophy of "a level is a level", the unified advancement table and the CR system helps simplify that a little, because you're only dealing with numbers from 1 to 20. You can, of course, achieve the same effect with XP, but you'd need a table to tell you how much XP equates to what character level for each class, and each challenge will need to state what XP range it is suitable for. I don't think an inexperienced DM needs to deal with that kind of additional complexity.