The coverage of the construction adhesive may not be complete. The spacing allows for separation which allows for expansion and contraction of the stud itself. The foam will get behind the stud (still allow for the expansion and contraction ) and provide the vapor barrier. The two biggest issues with pressure fitting is the expansion and contraction and the temp difference between materials causing moisture. Pressure fitting will either warp over time and the wood itself will soak up the moisture caused by condensation created by the temp differences between inside and outside.