I would guess that fighter is the single most used multiclass across all my toons. In it's pure form, typically fighters only have a few flavors. You have a stunner, a str based 2H with a monster token that flattens everyone, or you have an expertise, high AC and super-buffable dexer. As a multiclass, it's basically shield weap & armor profs, hitting 16 BAB or better, the most feats, & decent HP.
I've used barb as a dip for 2 builds. Pure class, the barb equates to a few flavors that can be interchangable: high DI/DR for both physical and elemental, usually 2H due to it's ability to really max the damage out, the best HP you can get, and the roar of coarse. There are a few racial trick combos out there too, because of the con base. As a multiclass, you still get the rage... so dipping 17 or 14 or something can still be useful (even when mixing with caster, it can free up a couple of lvl 2 or 3 slots). Str and con boost as a free action, multiple times daily is really nice. You get the base weap and shield profs (often mixed with clc as multiclass, you get plate and shield anyway, but you still get weaps). 16 BAB or better, uncanny dodge, and even some basic barb DR for 1,2, even 3 points.
I group bard and rogue together on my list of 'useless pure crap'. I've tried a number of times to plan and build a pure bard, but I just get bored. You fill up all the feats with the stuff you ABSOLUTELY need, like epic disc, maybe taunt, concentration, like you take everything you want. Then you STILL have 4 or 5 epic feats available. At this point, I'm taking epic energy resist or something nice, but really not a defined flavor. To me every bard is the same deep down, with some really soft personal choices for feats on top. Rogue, pffft. Basically you are trading an extra 5d6 sneak for no disc, 3 base attacks, the crappiest saves ever, and the ability to scroll (which you can do with 5 rogue, or numerous other classes). There's no point in a pure rogue right now.
You might ask 'What *should* happen, oh knowledgeable and wise Bargeld?' To me the answer is simple, leave it alone. If you feel the need to add another possible build out there, by all means, figure out how to purify a rogue, but we aren't lacking without it. Seems like development for development's sake, opening the use of more time and effort on a new project, rather than working on something existing (cough stun fist nerf cough).
Just my un-erring $.02
