The rest of us live in a world with dust ... I clean my desktop (and laptop, if I've used it) every month because I have pets and a very dusty room, doesn't matter how often I vacuum the carpet. Without fan filters, like with my old Antec 900 case, the dust was unmanageable.
For most people, cleaning every 6 months or so should be fine, so watercooling doesn't really involve much more maintenance than air, what little there is is certainly outweighed by the advantages (when used wisely, i.e. not in the OP's system).