Ah, leave it on then!
Re the watercooling questions, idle temps might not be much lower than a good air cooler, but your load temps should be lower with water.
For me, irrespective of the performance benefits, watercooling is another thing for me to mess about with, and I enjoy it! Plus, it looks nice!

A carefully chosen kit of components should out perform an air setup every time, although the gap isn't as big as it once was due to the more efficient thermal design of CPU's these days. Having said that, the latest generation of GPU's seem to kick out some heat, and the quad core CPU's are more of a cooling challenge, so watercooling might make more sense shortly.
A watercooling system is a closed system, so changing CPU's and/or GPU's shouldn't mean you having to drain and refill the system every time, as long as your cooling blocks are compatible with your new kit. For instance, I'm on my 3rd CPU in 5 months, and I haven't had to drain the system to change CPU's. You only need to do that if you need to change one of the blocks, the pump, or a radiator or something. Apart from that, no other components you're likely to change will need you to break the loop down, as long as the new kit fits your existing blocks. All that would be required is a clean and re-application of thermal interface material.