OT: D&D and multi-classing.

I think there are several ways to benefit from multi-classing before level ten (though your example of progressing on to a Prestige class is often a good option if you want something different). As I said previously, you can make a powerful warrior out of a Cleric by starting him with a single level of Barbarian. Or make for a very versatile dex-based fighter by starting with three levels of Thief.

Spell-casters often lose out the most by multi-classing, if they see spellcasting as their primary function. But if their "other" class is their main focus, then a few spells can oftern be a real boost.