From my experience, the usefulness of the fretless for ear training is that it makes you play the tune in your head trying to imagine it as precise as possible. When playing a fretted instrument, I often reduce to thinking in terms of fingerings. Playing the fretless bass is more involving. I'd put it somewhere between playing an instrument and singing. When you learn a song by ear on a fretted neck, you only nail down the correct notes and here you go, the rest of the work is purely mechanical. On fretless, you're always nailing down the correct note. There is a lot of room for error; the slightest change of angle in your finger position can ruin your intonation. To prevent this, you have to always be there.
However, I think playing fretless bass alone is not enough for successful ear training. You should know your intervals etc. This is better done on an ordinary neck or keyboard.
After all, there is a lot of applications that will help you with it. It's quite possible that identifying intervals while commuting to work on a bus an half an hour daily will help you more with ear training than playing a fretless. Who knows.