Wet transfers are prefered if you want to visualize proteins of high moleculacular weight (>100 kDa aprox). Proteins lower than 100 kDa can be visualized using dry or semi dry transfers with no problem.
I agree that wet transfer is preferable if you want to transfer high MW proteins (>100 kDa). Wet transfer can also offer improved signal/noise ratios - this is dependent on the target protein and antibody. Thus, if semi-dry transfer is not working well, wet transfer may improve the signal detection.