Let's talk about the people who had better leave the fretless bass alone.
Let me start with the Late great
Rick Danko, of The Band.
Now don't get me wrong, Danko was a good bassplayer, inventive and original. But only when playing either his Fender Jazz Bass or his Gibson Ripper fretted basses. I have seen 1974 footage of The Band supporting Bob Dylan and EVERY DAMN note Danko played on that fretless Ampeg was off, there was not one note fully in key.
The most horrendous part was when they played "The Load" which normally has one of the best basslines played by Danko, it had my toenails curl upwards when I heard him maul that otherwise wonderfull part on his fretless.
Then there's
Sting 
As with Danko, Sting is another superb bassplayer with a destinct style and tone. The problem with him on the fretless is that it both affects his singing and his playing. Sting is a Double Bass player so he probably thought that a fretless bass would be a natural progression but it wasn't. His fretless playing is off key and his singing (based on the root notes he played) is affected by that.
Eventually Sting wised up and began using an electric Double bass for live performances, which greatly improved his singing and playing.
