Which snails do you have? You cannot see their slime trail that easily. I keep lots of snails and never see any slime trails at all. If they appeared to secrete heavy amounts of slime, then it may mean something is wrong with your water conditions that is stressing them.
I keep ramshorns and bladder snails, and used to have nerites (before I accidentally killed them with my praziquantel treatment). These ones do clean the glass efficiently. Apple snails and MTS on the other hand do not and seem to prefer food leftovers and decaying plant matter.
Actually, all snails in my opinion, are valuable inhabitants of every tank. While many people do complain of their potentials to overpopulate the tank, it is the aquarists' fault for the most part because it indicated overfeeding and inadequate maintenance on their part which they fail to accept.