There are many very good reasons to use Air cooling instead of water.
There are also many very good reasons to prefer water.
People have reasons to prefer ATI or NVIDIA, Intel or AMD.
There are some good reasons to choose an Apple computer too, and many people choose them. A lot more choose more traditional systems, also for very good reasons.
I've even heard that there are good reasons to choose Windows over Linux, although I haven't heard any myself.
Point being, there are a lot of choices to be made about computers and good reasons for EITHER choice for any option you can think of- just depends on each person's needs, desires and preferences.
I choose watercooling for everything I can for two main reasons:
I enjoy the process
Safety
I have had quite a few parts damaged due to fans that failed.
I have never lost a part from water cooling that was NOT due to my own choice to experiment or leave a minor leak for a while.
I use H2O on every LAN rig I build (currently 2) and every computer I personally have running, except for initial testing purposes.
5 water cooled machines and one in the house on air - an email/web browsing station.
I have air cooling parts for one socket A system and one socket 939 system.
But those are MY choices.