Hi Bedimpediment,
5 sided mattress protectors only protect the top surface of the mattress.
A mattress encasement that is designed to help with allergies and dust mites will protect against dust mites on all 6 sides. They are meant to be semi permanent and remain around the mattress for the long term. If you choose an allergy encasement that is breathable (which is the ideal) then it wouldn’t be waterproof or water resistant and you would still need a mattress protector for the top surface of your mattress to protect against the fluids we release each night or accidents and which can be removed and cleaned more easily and on a more regular basis to keep your sleeping surface in a cleaner and more hygienic condition without having to remove the allergy encasement.
If you choose an allergy encasement that has a semi breathable waterproof membrane on all sides then you wouldn’t need a separate 5 sided waterproof or water resistant mattress protector since it would already be waterproof but it may still be a good idea to add a non waterproof protector for the top surface that can be more easily removed and cleaned. Membrane types of mattress encasements are much less breathable and you would have a more permanent “semi breathable” layer all around your mattress so for allergies I would lean towards a more breathable encasement with an additional water resistant or waterproof protector for the top sleeping surface.
In the case of a mattress encasement that is meant to protect either you or the mattress against bed bugs (along with dust mites) then they are normally the membrane type with a special zipper that keeps the bed bugs either in or out of the mattress and these are also meant to remain on the mattress permanently. This would allow less airflow through the mattress but it’s can be one of the tradeoffs involved in fully protecting a mattress against bedbugs or protecting you from any bedbugs that are already inside the mattress. In this case since they already provide 6 sided protection against liquids then you may wish to only add a non waterproof protector for the top surface that can be removed and cleaned on a more regular basis since the bed bug protector would already protect against liquids.
There is more about mattress encasements (either for allergies or bed bugs or both) in post #2 here.
Your certainly right that Full XL seems to be a dying breed.
Phoenix