Personally, I think if you're diving in the ocean where there are swells I think it's always a good idea to have a snorkel. You never know when you're going to end up in a situation where you're out of air and may need to make a long surface swim while having to deal with the swells. Granted if the swells are too big, a snorkel probably isn't going to be much help.
I think that diving in waters where you don't have to deal with swells (lakes, springs, quarries, or even protected lagoons), you can do without a snorkel. And, there are even situations where snorkels can be a hindrance. For example, swim through which don't have a lot of clearance (aka, the swim-throughs at Devil's Den Spring).
Just my two cents.