In other words, you're looking for a level of specificity that I neither want nor impose on my players. A base class is far too big a part of the game to confine to one specific archetype, so I define paladins as I define clerics, with the obvious addendum of "But more fighty than the cleric." In other words, yes, a holy warrior. Beyond that, a paladin player is welcome to define her paladin as she wishes.
If the flexible paladin sounds like an archetype that can be arrived at via multiclassing...well, barring smite evil and a few other paladin features, yeah! Considering some of D&D's other classes, I don't see any inconsistency.
Why do we need rangers when the naturey-warrior archetype can be accomplished by a multiclassed druid/fighter? Why do we need bards when the jack-of-all-trades archetype can be accomplished via wizard/rogue/cleric multiclassing, plus a musical instrument? (Or just mage/rogue, depending on edition.) Why do we need druids when the nature-priest archetype can be accomplished by a cleric with the nature-y domains? Why do we need LG-only paladins when the Galahad archetype can be done with a LG cleric/fighter multiclass? Come to think of it, why don't we all just play the three generic classes from the 3e Unearthed Arcana, and fluff to taste? No need for all of these hybrid and specialty classes, right?
Answer these questions, and you just might discover what niche the flexible paladin fills. Hint: It's as much a chargen/gameplay niche as an in-game niche.