Originally Posted by
Sierra
I think it more refers to how much excess chain it can wrap.
The wider your gearing range, the more you will need to wrap.
Ah yes, that makes sense. In my velo-illiteracy I thought cage was refering to the parallelogram-thing which would allow more movement across a larger cassette. But a "long" cage would be able to take up more slack in the chain when going from largest to smallest sprockets.
So a measely 10 speed would not need alot of range and a sort cage derailleur should suffice.