In the case of neoprene cement like what is used by DUI and Harvey's, it does soften the glue by loosening the bond between the neoprene particles in the glue.
But you're right that it leaves a residue that, if not cleaned properly, can keep those glued from sticking. OS Systems has promoted the used of silicone since 1981 and it works so good that several drysuit manufacturers have tried to get the same glue but have failed because it is a proprietary product.