Truthfully, there is no reason to buy snorkel/mask/fins (any or all) if you think you're never going to use it again.
You'll save a few bucks, and you don't have to lug it around. The equipment you'll be renting (or getting 'free' with your excursion) will work just fine.
And the sanitization question: We have that 'ohhhh that's gross' mentality on a lot of things, but for whatever reason choose to ignore other things. For example, you may eat locally and never give a thought to the cleanliness of the dishes and glasses you use. You certainly handle money and touch doors every day, a couple of the most germ filled things in life. The snorkels are clean enough.