The warranty argument for a cotton encasement
Allergy is the obvious reason to buy one. It is not the only one, and for most households it is not the expensive one.
Almost every mattress warranty is void the moment the mattress is stained. Manufacturers say so in plain language, and they enforce it. A single spill, one accident, one night with a sick child, and a ten-year warranty is gone. An encasement is the only thing standing between those two outcomes.
Deluxe organic cotton, fully zippered, machine washable, and it keeps the mattress underneath in the condition the warranty assumes.