Gaffspan,
Yes, ABS plastic is affected by sun light. But! It isn't a fast process and I'm not sure I'd worry about it too much. All Plastics have an ingedient that keeps it 'soft', pliable. It eventually evaporates leaving it brittle (degradable, right?). It depends on the particular type of plastic, but it doesn't happen 'over night'. If you leave your boat in ~direct~ sun light all the time it will 'degrade' faster than if you don't.
The draw back in painting your boat is that it adds weight and drag. If you're into 'fast' sailing, it might affect your speed somewhat.
- 'Doc