The great debate - Is lath and scratch coat necessary on thin limestone veneer?

we are about to get thin limestone veneer installed. stone is approx 1.5" thick (varies up/down some). Contractor is saying lath and scratch coat isn't needed if you use the right thin set. i thought it would be a conservative way to provide a nice strong hold so i was asking for it. So... is it a nice to have or is it necessary for thin limestone veneer on an interior fireplace wall 10 ft high?