Its not a need, but it would be stupid not to. BASE jumping is a sport that kills complacent participants very quickly. There isn't alot of room for error. BASE isn't regulated, there are no licenses, no certifications, and nobody holding a jumpers hand to keep them safe. But to gain the necessary knowledge of canopy skills, emergency procedures, body positions, and so on, I wouldn't mentor anybody with less than 200 skydives. And no organized event anywhere in the world would let a new guy jump without having a few hundred logged and signed skydives, has taken a BASE course, or at least has a well known mentor. I remember looking back at my first 50 or so BASE jumps and thinking, "how the hell have I lived so long? I didn't know skit back then." And I have a feeling I'll look back again at this time period and think the same thing. You're constantly learning.
With that said, I think skiers make great skydivers and BASE jumpers. They understand how to control their body better in subterminal air, making them more stable then most new jumpers. And they are more aware of what their feet and hands are doing.
If anybody is in the NW and is looking to get into skydiving, PM me. I can help. It's a rad journey that can lead to all sorts of places.