For most resort riders, water-resistant boots are perfectly fine. They have a treated outer shell that sheds light snow and splashes, which covers you on typical groomer days and even moderate powder. If you're mostly riding lifts in normal winter conditions, these boots will keep your feet comfortable without the extra cost of a waterproof membrane.
The limitation is that water-resistant boots can wet through during sustained exposure. If you're post-holing in deep snow, sitting on cold lifts during heavy storms, or riding in rain and slush, moisture can eventually penetrate to your liner. Once your liner gets damp, your feet get cold fast.
Stick with water-resistant boots if you ride mostly fair-weather resort days and want to save some cash. Just keep in mind that the treatment wears off over time—reapply a waterproofing spray annually to maintain protection. If you regularly ride in wet climates or venture into the backcountry, consider stepping up to a waterproof membrane instead.