Also, I find that with the BASE classes, multiclassing rarely increases power. More often than not people end up slightly nerfing themselves by stalling their core class progression.
Your example with a Barbarian/Fighter is incorrect, that is not strictly superior to a pure Fighter. The character has effectively trading 1/2 of a fighter feat for what he gets as a level 1 Barbarian. They've also incurred an XP penalty (or burned their favored class use). And really that sort of trade off makes sense.
The biggest "broken" bits of multiclassing BASE classes that I have seen were the 3.0 Ranger class (fixed in 3.5, thankfully) that got way too much for just being level 2 and people who take a level of fighter just to get all the armor and weapon feats. The latter I have patched with a house rule saying that you don't get any additional weapon/armor feats for multiclassing. It's a good rule that works very well in practice and it makes a lot of sense with the system as it is. You don't get to retroactively pick up [4xBase Skills Points] for taking a different multiclass either.
Arguably a Barbarian's base movement bonus could fall under this as well but I haven't rule that.
You might find a few examples of where a multiclassed character has a slight advantage at a certain level but generally I think it averages out. It's also the case that certain base classes are just more powerful, overall, at certain levels than others, but that averages out as well.
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Where I've seen big problems with allowing all sorts of multiclassing is when DM's let players take any prestige class or base class in any book they've found. That seems like the core issue here. I don't understand why a DM would do that. Many of the prestige classes and variants are outright broken. Even in the core books like the "Complete" series. I take a single glance at a lot of those classes and all I see is "broken!". Furthermore, when you allow any prestige or variant class in the book into your campaign you're just not doing a good job of vetting what matches with your campaign style and so yes you're going to get characters that are all over the place.
So yes, if you let your characters dabble in any PrC they find in any book they will end up all over the place and broken-good. Simple solution: vet what PrC's you let them take! Use a little common sense about what classes fit your world and how your players use those PrC's. Problem solved.
Your example with a Barbarian/Fighter is incorrect, that is not strictly superior to a pure Fighter. The character has effectively trading 1/2 of a fighter feat for what he gets as a level 1 Barbarian. They've also incurred an XP penalty (or burned their favored class use). And really that sort of trade off makes sense.
The biggest "broken" bits of multiclassing BASE classes that I have seen were the 3.0 Ranger class (fixed in 3.5, thankfully) that got way too much for just being level 2 and people who take a level of fighter just to get all the armor and weapon feats. The latter I have patched with a house rule saying that you don't get any additional weapon/armor feats for multiclassing. It's a good rule that works very well in practice and it makes a lot of sense with the system as it is. You don't get to retroactively pick up [4xBase Skills Points] for taking a different multiclass either.
Arguably a Barbarian's base movement bonus could fall under this as well but I haven't rule that.
You might find a few examples of where a multiclassed character has a slight advantage at a certain level but generally I think it averages out. It's also the case that certain base classes are just more powerful, overall, at certain levels than others, but that averages out as well.
...
Where I've seen big problems with allowing all sorts of multiclassing is when DM's let players take any prestige class or base class in any book they've found. That seems like the core issue here. I don't understand why a DM would do that. Many of the prestige classes and variants are outright broken. Even in the core books like the "Complete" series. I take a single glance at a lot of those classes and all I see is "broken!". Furthermore, when you allow any prestige or variant class in the book into your campaign you're just not doing a good job of vetting what matches with your campaign style and so yes you're going to get characters that are all over the place.
So yes, if you let your characters dabble in any PrC they find in any book they will end up all over the place and broken-good. Simple solution: vet what PrC's you let them take! Use a little common sense about what classes fit your world and how your players use those PrC's. Problem solved.