Our Thoughts: Unless you purchased a crib mattress that is not waterproof, a crib mattress protector is generally not a necessity.
In our case, we purchased the Naturepedic® Organic Cotton Ultimate Baby Crib & Toddler Mattress, which is waterproof and features dual firmness to provide a firmer side for infants and a softer side for toddlers; however, because we found the firmness (essential for an infant-safe sleep environment) to be a little too firm, we supplemented the mattress with a TL Care® Waterproof Crib Fitted Mattress Cover Made with Organic Cotton in Natural. While Sydney was largely unable to verbally corroborate our intentions, we believed the quilted organic cotton surface safely softened the sleep surface a bit.
Sh!t That Worked For Us 