Ranger REG: I don't think your logic holds up.
"But if you want to introduce prestige classes into the game, then you have to make prestige class appealing that they're willing to multiclass to it. You have to ask, "what is so good about a prestige class that I'm not getting from my core class?"
First of all, you have to ask why you want to introduce prestige classes into the game. What _IS_ so good about a prestige class that I'm not getting from a core class? That is the question you should ask before creating any class, not afterwards to justify the creation of the class.
You don't say "Gee. I want to introduce an archer class to the game. Fighters are already good at archery. In order for people to want to take it has to be better than fighters. What sort of things do I want archers to do better than fighters." You should say, "Gee, there are some archery things that no class can do. I want archers in my game. Should I create an archer class?" And the answer should probably be, "Fighters are already supposed to be good with weapons. Rather than creating an archer class, lets just extend the fighters custumability by adding more archery feats to thier bonus list."
Secondly, why do you have to make the class more appealling than a core class? Don't fighters occassionally take levels in rogue? Is this because rogues are more powerful than fighters? Or is it because a few levels in rogue lets them diversify into areas outside the sphere of 'combat with weapons'? Is combat with bows outside of the sphere of fighter specialization? Are they not allowed to take 'weapon specialization: longbow'?
Are they not allowed to take archery bonus feats?
The character kit is free, so anyone with a character kit tacked on have a better advantage over regular character without a kit."
That is arguable. The idea behind the kit was that a person with a kit would not be advantaged over someone with another kit or no kit at all. It didn't always work out that way - a consequence of there being so many kits designed with so many different philosophies - but that was the idea. Each kit was supposed to balance a very small advantage against a very small disadvantage. PrC's were notable in NOT being created with this idea in mind. They were designed to be unbalanced with core classes right from the beginning, so any one with a PrC was supposed to be by definition have an advantage over a regular character without a PrC.
While a prestige class is also tacked on, you do so by spending your character level (and XP) toward a level in that class, so it is not free.
Errr... come again? You are spending your character level (and XP) toward a level in some class right? That XP is going to be spent somewhere. It is not a burden to gain a level. (Oh, dear, I have to advance a level. Woe is me.) So the choice is between spending it on a level in this or a level in that, and if the PrC carries no significant disadvantage why not take it? (Gee, I that PrC forces me to do extra damage with my attacks, and forces me an extra feat each level which adds to my character concept. I don't know, should I take it?) I mean, the whole thing that triggered this rant of mine was that to top it all off, PrC's don't cost you any extra XP when you take them.