Classes, classes, for the love of Pelor, CLASSES!
My reasoning is that a class can carve out the niche feel of an archetype better than a basic class and themes/backgrounds.
The thing is that the multi-classing approach has never given us a good and balanced fighter-mage. We had the one that was way better than a fighter or a mage in 1e and 2e (the balancing factors being largely irrelevant), the one that was way worse than a fighter or mage in 3e (even if you were very careful and took 'patch' prestige classes), and the one that was either not much of a fighter or not much of a mage (4e multiclassing); the only multiclass version that's not horribly imbalanced is a 4e hybrid (though it's on the weak side because there's little synergy).What I think we don't need is a bunch of classes that fill space already represented by other classes; if a class can't have a distinct identity, I think focusing on building it with a base class theme and background is better.
(For example, I really think the swordmage archetype is better served by a fighter/wizard [or ranger/wizard or fighter/sorcerer or... etc], or the swashbuckler archetype by some combination of rogue, fighter and ranger; otherwise there is too much overlap. The base system's classes should support such archetypes without needing artificial classes to patch the holes [see the 3e swashbuckler, beguiler, etc.- I have been told the Book of Nine Swords was especially egregious in that it made the fighter obsolete, but don't know that first hand].)
I agree and that's why I dislike having too many classes. A class is something I should get a very clear idea what it is by it's name, concept-wise. So I don't mind a fair amount of niche classes like Sorcerors, Warlocks, Assassins, etc. But what is a Runepriest? Or a Soulknife? Or the other multitudes of classes people reference around here from books I don't own.