They don't gain new abilities, no. They simply improve upon the old ones.
Regarding the other combinations, RDD gives very high strength and a quick accessible Devestating Critical. Weaponmaster gives a better overall chance of scoring a critical in the first place, whereas for the Pale Master's bonusses it's mostly in the AC bonus and immunity to critical hits.
The Shadow Dancer's far less combat-oriented and really does work best with a good sneak attack.
As a side note, I've never been a fan of monks and absolutely love the Shadow Dancer prestige class (and hate true sight, naturally, unless it's my own). It doesn't have the same kind of combat deadliness though, in this case.
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