Here are two scenarios that relate to this discussion.
Scenario 1
Player: My character is growing stronger in his faith, I think I'd like to try and learn some Clerical Magic to augment my crusade against evil.
DM: Ok, well, lets check out the Dual Classing requirements. Is your character Human?
Player: Yes.
DM: Well, that's good, otherwise this whole point would be moot. Now, does your character have at least a 15 in your class's Prime Requisite Ability; in your character's case Strength?
Player: Yep.
DM: Good, good. Now, does your character have at least a 17 in the Prime Requisite Ability of the class he's trying to become? In this case, since you'd like to become a Cleric, Wisdom?
Player: Oh. No. He only has a 13 Wisdom. I had hoped to only take a level or two of Cleric, because it would be both in character and a good mechanical benefit for the group.
DM: What do you mean?
Player: Well, I feel that my character has been a strong force of good in the world, and his God, Kord, holds an important place in his heart. He knows that he isn't the wisest person, but he has a good heart. He wants to help rid the world of evil, but completely scrapping his formitable fighting abilities would be out of favor with his religion's core beliefs. Thus, it would be much more in character to just take a little bit of clerical training, then return to the "front lines", as it were.
DM: Well, in order to take any levels of Cleric at all at this point, you have to have a 17 wisdom, and you're stuck as a Cleric until your character dies or retires. Also, while you were "training", you would lose your Fighter THAC0 and all weapon and non-weapon proficiencies. They would be replaced by the THAC0 and proficiencies of the Cleric class until such time as your Cleric level excedes your Fighter level. Also, remember that your XP chart is different for Cleric, so keep a close eye on that.
Player: But my character is a level 15 fighter! I've been playing him for 4 years! It'd take nearly that long just to get my old abilities back. There has to be an easier way.
DM: Well, we could always try to make some kind of houserule, let me check the TSR BBS and see if anyone has ever attempted this before. Failing that, I guess I could send a postcard to Mr. Gygax to see if he has any other suggestions.
(Three months later.)
Player: Anything yet?
DM: Well, I came up with this fix. If you go on a quest to do (instert long and arduous quest here), by yourself, and sacrifice 40,000xp; you can gain the ability to cast Cure Light Wounds, Sanctuary, and Command once each per day.
Player: If that's my only option, I guess I'll have to do that.
Scenario 2
Player: My character is growing stronger in his faith, I think I'd like to try and learn some Clerical Magic to augment my crusade against evil.
DM: Ok, I'll try and work a chance for you to undergo some sort of training into an upcoming session. That way, at your next level, you can take a level of Cleric.
Player: Thanks.
Now, I much prefer the second scenario. The Unified XP tables make it feasable and workable. There are those that prefer the system described in the first example. That's fine. Whatever works for them, I guess. More power to you if you enjoy houseruling any non-standard character.
If we could just ditch the silly preconcieved notion that Class = Profession, and move on to a frame of mind where Class = Abilities Learned, we could move on into a much less angry world.
You can play a "Holy Warrior" by any of the following combinations (and many, many others):
1) Paladin
2) Cleric
3) Fighter / Cleric
4) Fighter / Paladin
5) Fighter / Ranger
6) Fighter
7) Knight
8) Bard / Rogue / Druid
Ok, so maybe that last one is more Holly Warrior than Holy Warrior, but I hope my point is being conveyed; in 3.x, your class isn't your profession, it's just one part of your abilities.
To paraphrase-quoth the Order of the Stick -
Elan: So, Miko, did you take levels in the old Samurai class or the new one?
Miko: I did not take any levels in any "Samurai" class.
Elan: But you said you were a Samurai.
Miko: Yes, that is my position in the social class system of my homeland; not my core class. My class is Paladin, not excluding the Monk training mentioned earlier.
Elan: So then you took levels in the Master Samurai prestige class!
Miko: Why is it so difficult to believe that I can be a samurai without any levels in a class with the word "samurai" in the title? Can there not be facets of life that are defined by something besides one's classes?
-TRRW
fake edit - I hate Miko Miyazaki with a firey passion. Hate, hate, hate, hatehatehate. I'd show you the drawing I had Rich Burlew do of her on fire, but it's currently on my wall and not easily scanned.