There's nothing in the rules that says a prestige class -must- be 5 levels, and I can see why people would want to be able to stay in a class for longer. Of course, in D&D most prestige classes should have more than 10 levels by that logic. In fact, in my own games I've added a few 15-level PrCs.
But WotC is unlikely to add 8 or 10 level classes to D20M (though anythign is possible). I remember once asking Skip williams if there was any reason a D&D prestige class couldn't grant an odd number of skill points per level. He responded that there wasn't... except that it created a new "tool" that was going to be difficult to balance.
It's very hard to keep prestige classes balanced. By having them all be 5-level classes, you can more easily rate their balance with each other, since it's a more apples-to-apples comparison. Whenever you change some element (a new attack progression, an unusual number of skill points per level, a non-standard number of levels in a PrC) you have to guess how that's going to effect overall game play.
A 10-level PrC gives you more payoff for meeting the prerequisites, making it more appealing. But you can't have much higher prerequisites, because then you can't get into the class early enough to finish it by level 20. Most "logical" advanced or prestige class combinations have some wiggle room anyway... so you don't -have- to follow the most logical path in order to top out the classes.
That said, if a 10-level PrC that was well desgiend and fit it's own concept well shouldn't ruin anything. If it's well done, it's well done. Especially since there are a lot of them, you can judge them against each other, establishing a new "balance."
Play Well!
Owen K.C. Stephens