Its not a mere formality it is unfortunately one of those things that makes satellites expensive. However I wouldn't be surprised if all kinds of nominally incompatible materials have already been exposed in cubesat missions. EDIT This report might be useful background: NASA contamination control report 1996
It could in principle affect other parts of the cubesat or in the shorter term other cubesats in the dispenser or other systems outside a cubesat dispenser. A consequence of the dispenser being an enclosure is that there will still be some leak paths and these become the main out-gassing vent routes. The time for this to occur might limit the worst of the hazard to the host cubesat.
The mechanisms of adhesion to other surfaces are varied and include condensation on cool surfaces and also UV ionisation and electro-static return to a surface not on a direct flight path. EDIT The link I added above has a short section on this, section 220.127.116.11.2.2, figure reproduced below:
Note however the reference goes on to say that this particular mechanism is believed to be much more of a problem at GEO altitudes because the plasma screening distance is much greater than at LEO.
Outgassing products can be water vapour, most solid objects have a dose of trapped moisture, or something more destructive. See this article http://space.skyrocket.de/doc_sat/hs-702.htm regarding ouggassing from solar arrays. It says:
The first version of the 702 used solar arrays with concentrators. These concentrators tended to early fogging, as due to an inherent design flaw the outgassing of the solar cells was higher than expected. This fogging lead to much reduced lifetime.
The latter satellites all weighed several tonnes and I suspect cost over a billion $ between them.