I have one for the V850, but I haven't used it yet. Although it's a lot slower method, It's supposed to give better results. I have some 35mm b&w negatives that I'd like to take my time with and get the best scan possible. I would like to print them on 13"x19" paper.
Eric at the Film Photography Channel does a comparison between dry and fluid mounted scans (comparison starts @ ~12:50 mark):
My experience has been that if you're having difficulty keeping the film flat, then fluid mounting solves this rather quickly. This is more of an issue with MF film, as opposed to 35mm film.
ETA: I've also found fluid mounting the only reliable method to use when scanning older films that don't fit in the supplied holders (e.g., 110 or 127 film when arching old family photos).
OK I found the comparison video. It's from Nick Carver. He compares the Betterscanning fluid mount vs the Epson fluid mount as well as dry mounts on a V750. He found the Betterscanning wet mount the best. Interestingly, he found the V850 dry mount film holder the second-best, better than the Epson wet mount. Here; is his video.