I think it is in the inaccuracy of the notes
Seriously, though. The warmth of a string section comes from the fact that each of the individual players is minutely out of tune with the others, creating a natural and welcome chorusing effect (if you tape the same string player 20 times, it doesn't sound like a string section). There is a 'human' quality that this inaccuracy brings to the table that adds considerable character to the notes played on the fretless and which is missing in a fretted bass. Also, if you listen to a double bass soloing, when they go into thumb position, the inaccuracy in their intonation brings a huge amount of tenion into the note, like a singer reaching for a note that is out of their range. This tension, which is less obvious in an electriv bass but is still there, adds colour to the performance and, when it is missing, it is noiticeable. For me, there are several fretless players whose intonation is SO accurate, they lose this human quality to the detriment of the music.
All of this sounds like a justification for bad intonation and it is important that we understand that I am talking about barely audible inaccuracies not 'wrong' notes. My intonation is by no means perfect and I struggle with it every day but, the use of a fretted bass (with its built-in 'stabilisers' ) is, for me, a 'safe' option that has its attractions but, in the end, is not worth the price of that lost 'human' quality.
Does that answer the question?
For the record, I use fretless in rock, blues, funk, function bands, shows, big bands, small jazz trios and latin music. It always works. The only areas I see it losing out to fretted are in chordal work, tapping and slapping, none of which are a feature of my playing.