One use I found for the otherwise-mostly-pointless sniper turret is for beachheads when you are trying to kill something nasty under a level L3 or core force field, but you can't assemble enough firepower to clear it in one go (spec ops under a force field for example).
The trickle damage caused by the snipers prevents the forcefield from recharging, giving you enough time to rebuild your attacking forces.
Snipers also seem to be useful if you are being plagued with raptors... 10-20 clean them up at a fairly reasonable pace.