I just ordered a Cirrus Supreme-ES 11" split king mattress and Leggett and Platt designer series split king adjustable base. Woohoo!! My question is I’m worried that a mattress “cover” will slip off between the two beds and thought maybe I should get an encasement instead. Is this usually an issue or no? My other consideration is that my husband has allergies. Thanks for any advice! 
Hi zzzforme,
You can see some of my thoughts about a mattress encasement (which surrounds the mattress and zips up) vs a mattress protector (covers 5 sides only and mainly provides protection for the top surface and to some degree the sides) in post #2 here.
In most cases a mattress protector will do a good job of helping to control the dust mite population and prevent moisture and skin particles from entering the mattress (which dust mites require) and they can easily be removed and washed as well. Post #89 here has more information about the different types of mattress protectors and the pros and cons of each.
With existing allergies that are more severe then a complete encasement will offer better protection against dust mites and their “byproducts” which are one of the biggest allergens in household dust. The “membrane” type of encasements will reduce breathability (they are only semi breathable) and can increase sleeping temperature but are waterproof. The cotton or microfiber type that have a small pore size are more breathable but may also need a protector in addition to the encasement because they are not waterproof and are designed more for allergies than for mattress protection. The encasement in this case would go around the mattress and the protector would go above this.
A well fitting mattress protector shouldn’t have any problems with slipping off the mattress (even with an adjustable bed) and they are usually well secured to the corners and sides of the mattress (like a fitted sheet).
So the choice between an encasement and/or a mattress protector would usually depend on the severity of the allergies and the degree of control for dust mites and other allergens that are important. It would also depend on the type of protection you wanted for your mattress against moisture, liquids, and body oils and the tradeoffs that can be involved that are important as well.
Phoenix