Actually, it's worse than that. Windows should consume ALL available memory, since unused memory could potentially be used as cache. Same thing that Linux does - if you've got more commonly used disk files than memory, your memory should be full 100% of the time after your machine is running warm. This can only increase performance, there's no penalty for caching something from disk (if the disk would be otherwise idle).

It's kind of like claiming that disabling Intel's SpeedStep is a good thing, since it makes the computer run faster all the time...