Dead organic matter will accumulate over time in the sand, creating a rather toxic environment if left stagnant. This goes for all types of sand as well as for gravel, to a lesser extent. Finer grained and heavy sand gets compacted faster.
The sand snail species I keep are Thiara winterii and Thiara scabra, as well as Melanoides tuberculata. I find Thiara species to be somewhat more effective than MTS, although they reproduce more slowly. They look interesting as well.
I have 1-3 inches of sand in my tanks, and i never have a problem, as they all have snails.