Mostly a gimmick, although it has some practical benefits. Imagine a wireless pad on your desk at home/work: you just drop the phone there when you come and pick it up as you leave, without having to fuff around with connectors. For devices like N900, it is even more useful because it releaves the stress on the weak USB connector.

Other than that though, you are right. It does not save the number of cables or the space on the desk. Quite the opposite in fact.