I wanted to chime in on this one...
It is correct that the membrane-type diffusers do have integrated checkvalves whereas the membrane will collaps on to the supporting disc or tube (depending on the type of diffuser you purchase).
If you look at all of the suppliers, we still include a "Blue" Brady Check valve as a secondary check to the standard check provided by the membrane. This is due mostly to the seeping of the water through the slits and eventually into the air supply line.
Yes, eventually the water will be pushed out by the compressor but it will overwork the compressor unnecessarly until it does.
Most membranes offered by companies are now automatically cut with 500 micoron or 0.5 mm slits to allow for the smallest bubble size. As Rainman stated, the smaller the bubble the more bubbles are allowed the greater the surface area and the greater the lift.
There are reasons behind thing that have been done to systems to keep them as maintenance and trouble free and this is one of them. If I can make a recomendation, do not skimp on this one.
