Chun-tzu said:
That's exactly the point, actually. You're assuming that a Rogue 10/Wizard 10 is an effective level 20 character; I'm thinking it's not. Your best spells are 5th level, you have a modest selection of skills (only a handful of them maxed out), and you are probably not a major threat in combat compared to any single class character.
Hmm...a 10th level rogue with improved invisibility, stoneskin, haste, displacement, cat's grace, bull's strength, and shield who does an extra 5d6 damage and can cast teleport or dimension door to get out of any nasty situations isn't a potential threat in combat at 20th level? While his spells are not potentially the most powerful out there on the field of battle, they can enhance his own abilities greatly.