Originally posted by drnucheon
And yes, a 20th level wizard could outclass a rogue too - but they're not likely to if their job is primarily to be an arcane spellcaster. If they focus all their spells on substituting for a rogue, they will be a poor wizard.
This is the entire point we're trying to make. Yes, from a certain point of view a Wiz20 could out-rogue a rogue. So could a Wiz15/Rog5. If they do so, they'll both be giving up other wizardy areas. However, this affects the Wiz20 much less than the Wiz15/Rog5, and the Wiz15/Rog5 isn't really getting anything out of his 5 Rogue levels. Whether or not you play him as a rogue makes no difference. You could play a Wiz20 as a rogue, and be more effective in every way, unless you wanted to create some kind of invisible+ranged sneak attack one-trick pony, but even with ranged touch spells, this is hardly your most effective combat option when you have things like fireball and chain lightning available to you.
As I've said several times above, whereas your usefullness as a fighter or rogue increases even if you're not taking fighter or rogue levels, your usefullness as a caster doesn't.
So, a Ftr10/Rog10 doesn't just have the ability of a 10th level fighter, plus the ability of a 10th level rogue. Because there's synergy between his two classes, he has something approximating the ability of a 16th or 17th level fighter, plus something approximating the ability of a 12th or 13th level rogue. The reason for the imbalance between the two is the small amount of skill points that a fighter gets.
OTOH, a Wiz10/Rog10, like the Ftr/Rog has approximately the ability of a 12th or 13th level rogue (ignoring for a moment the issue of ability bonuses), but only the ability of a 10th level wizard, because there is no synergy there.
Perhaps I would be more likely to believe you if you showed me what a 20th level rogue could do that the Rog/Wiz (or the Arcane Trickster) could not, instead of just baldly saying "it can't contribute as well as a wizard".
The Rog20 can do lots of things that the Wiz/Rog or the Trickster can't. His sneak attack is +10d6, and he's likely to be much more able to withstand being in a flanking position in combat in order to be able to use it. He also has a better BAB, so he succeeds more often in hitting with it. He's likely to be able to tumble much better, in order to get into position to flank without drawing AoO's. The opportunity to do +10d6 damage 2 or 3 times around with a full attack action, or even once a round with a standard action (assuming the rogue is trying to spring attack, or something), is nothing to scoff at. Sure, the Trickster has a 7d6 sneak attack, but, as I've said above, he's going to have a harder time using it. Plus, the extra 3d6 is a big difference, even over the length of one combat.
The straight rogue also has more skill points than the multi, so he's going to be better at most of his rogue skills and have more of them. Unlike the multi, he doesn't need to rely on a limited number of spells to be effective. He can hide, or pick locks, or disable traps, or spot hidden creatures as much as he wants. Sure, the Wiz could dimension door past a locked door, but how many times can he do it? 4-6 times a day, and that's if he blows all his 4th level spells on it (remember that he also needs improved invis, and possibly poly self at this level to be an effective rogue, too). Plus, when the Rogue picks the lock, he can also get his friends through the door at the same time as him, instead of foolishly leaving them on the other side, like the guy who dimension doored.
The Rogue also has improved evasion (which the Rog10/Wiz10 probably also has, but not the others), which means that even if he flubs and fails his reflex save against a fireball or dragon's breath or something, he's not toast. He also has other cool special abilities. Skill Mastery means that he doesn't have to worry about flubbing a roll on his skills anymore. Opportunist can grant him still
another sneak attack if he's flanking in combat - there's another 10d6 of damage. Crippling strike means that for every two sneak attacks that he does, his opponent gets a -1 to hit and damage. Defensive Roll can save his ass from dying. Or, he could choose to have bonus feats instead of these, in order to increase his combat ability.
All of these are areas in which the straight Rogue outshines the Wiz/Rog or the Trickster. Now, granted, the rogue might feel outshined on those occasions when the wizard does choose to use his spells to duplicate rogue abilities, but how is this any different from the fighter feeling outshined when the wizard busts out with a chain lightning or a horrid wilting? Both characters know that they're being outshined at the moment, but that they have a steadier rate of long-term performance.