It doesn't make them nearly as powerful as going up in levels would. There's no 4th level spells, there's no jumping like the Hulk, there's no +20 BAB, etc. A fighter 6/Rogue 4/Duelist 4 has a +6 BAB (same as f6), 6d10 hips (same as f6), a +5 Fort (same as a f6), a +4 reflex (same as a f6 with...