It should b e remembered that, during the relatively short time the compressor is actually used for spraying the average model, the possibility of moisture build-up, or 'spitting', is minimal.
These compressors, and air brushes, were originally designed for use in graphic and art studios and similar uses, where they'd be running virtually continuously, five or six days a week, with an actual spraying session perhaps taking minutes, not the few seconds per 'burst' in modelling. This would cause a build-up of heat, and therefore the associated moisture problems. This rarely happens in modelling and, although it's a sensible precaution, the water trap is not really needed under average conditions of use.
I've used my very basic, little compressor for five years now, and have never had a mositure or spitting problem. If I was to run it for continued periods, however, I would expect this problem.