It sounds to me that what's actually going on might be a common fear when first playing the fretless. It seems to crop up once in a while in threads. That is, that fretless seems like it will be hard to do and take a long time to get the hang of. I'm sure this may be what partly fuels the lined fretless market. It's actually not at all difficult, and IMHO a lined one wouldn't make your life easier if you really wanted to go over to playing an unlined fretless. I've always thought it better to jump in feet first, not worry about it, and after not too long your brain, hands and fingers will start to synch up for fretless playing. It won't take all that long to do, believe me.