Umbran said:
Please don't be so heavy handed with the absolutes (like "always"). What is favorable depends strongly upon the type of game you play, and how you like to play it.
If the semantics bother you that much, then you can substitute always for "any standard-style d20 campaign".
d20 mechanics focus on combat, much more than skills or interaction. Fighting and slaying other creatures are at the heart of it. Hence, a single-classed spellcaster will do better because they can deal out more damage with their higher level spells, hence they'll "always" be more favorable.
Other DMs may expect the PCs to have a more broad skill base, so that they call for more of the party members to be able to hit lower DCs. They may tend to wear the party down with a large number of low-CR encounters instead of hitting with only a few high-CR challenges. For these games, single-classed characters are not necessarily optimum.
Except that the way the d20 system is designed, these games are far from being the norm. For one thing, the generic party mix (of single-classed characters) can easily handle the above scenario due to the fact that the diverse members of the party tend to cover most skill applications relatively well. Likewise, throwing many low-CR encounters at a party doesn't necessarily favor multiclassed characters, as low-CR creatures will consume less resources from the party; a high-level wizard most likely will not waste his
Power Word Kill on a group of orcs, even if they're the fifteenth group of them he's encountered in a row (since, quite likely, they'll have a hard time even hurting him, let alone the rest of his party).
There are many gamers who decide they want their characters to be the absolute best at whatever their shtick is. And in Tri-Stat, each player may do so, so long as the shtick is reasonably narrowly defined.
The difference comes when not sticking to a narrow field though. d20 penalizes you for having a wide range of abilities much more than Tri-stat does.
But, if thorough maximization in a narrow field is not your character concept, or if such maximization is not called for by the challenges presented by the DM, there's little issue here.
There's a great issue if it's in a d20 game. Tri-stat is good because there is, relatively speaking, little in terms of character advancement over a campaign (especially compared to the points used to build a character in the first place). In essence, the majority of a Tri-stat character is done when he's first made. This means that branching abilities out have less impact because less is lost, being closer to the end of possible character advancement than the beginning.
A d20 character is only at 5% of his final characterization (assuming the standard 20-level progression) when he's created. A character who multiclasses is (in terms of class abilities) essentially starting over, and it becomes more and more expensive to do so, as each new level costs more. A 15th-level Fighter who takes his next level in Wizard is getting much less bang for his experience points than at 1st level.
