Flexor the Mighty! said:
I'd let a monk multiclass with a cleric, but a paladin cannot multiclass under any circumstances. Take one level of anything else and Paladin is closed to you forever. It's a heavy commitment, not one to be taken lightly. Play a Paladin for the class and what it means, not as a way to get kewl powerz.
This thread represents how far the idea of character archtypes has fallen with the current gaming community. alas.
Trouble is Flexor, the Paladin is already the most restricted class, in that they have only a single alignment open to them. By forbidding all multi-classing without closing off the class forever, the only real distinctions between Paladins are their Feat choices and Code (provided your DM has the idea of modifying the Code - there are clearly some out there who just follow the PHB). There is a tendancy to make the Paladin cookie-cutter.
Opening up the multi-classing rules to them gives them the chance to shine in particular areas. I agree that some might still be tempted to take the class for 'kewl powerz' but it doesn't alter the fact the Paladin lives by a Code, and someone who plays the class carelessly can soon find themselves requiring and atonement, or worse, losing those 'powerz' for good.
All told, I think they would have done better by saying that anyone drawn to the life of a Paladin tends not to multiclass, and that if they do so, it is only to better live up to the ideals of their deity or philosophy. In other words, all multi-classing is flavour-related but
not out and out prohibited.
Similar considerations apply to the Monk. The way they are described makes for distinct monastic sects and orders. These can clearly differ in how they approach the path of perfecting the self, and might well encourage the adoption of additional classes to fulfil that path, e.g. the development of ki powers modelled by taking levels of Sorcerer. Again, a brief descripion of possibilities would have made for a better application of the principles behind 3e.
I think one thing you have to bear in mind. With the 3e multi-classing rules, just because a character has levels in more than one class doesn't equate to them following multiple archetypes. Instead the multi-classing is a rule-mechanic way of describing what that character is and always has been. For example, the Paladin with levels of Fighter is still a Paladin, but he is more martially-minded than some of his brethren. Likewise the Monk with Druid levels may represent a sect that considers the perfection of self in light of the perfection and preservation of the natural order. The first is still a Paladin, the latter still a Monk, but they have their own spin on things. I think 3e should encourage such thinking, not simply rule it out of bounds.