No, they're not absolutely needed, but it depends on a lot of factors. What are you trying to do?
I have a skimmerless system, but it's coral-heavy. Lightly stocked with fish, lightly fed, lots of LR, religiously maintained, and a very biodiverse crew of detrivores. The average marine system would greatly benefit from a skimmer though I think. If you're just starting out, I would go with one. It'll give you a bit of a safety net.