I read a great article in Bass Player Magazine a decade+ ago about how theres a floor and a ceiling to all musicians. The ceiling being your best abilities and the floor being your worst. For example, you could be a guitar player and be great at soloing but not so great at comping. Your solo ability would be the ceiling and the comping would be the floor. If you wanted to be a better all-around musician in this scenario you would practice your comping more. The conclusion of the piece was that if you work on improving your floor then when you have an off-night of playing it won't be a disaster, it will actually still be pretty great.

Phish pushes the ceiling with their improv, and recently have been lowering the floor with some of the composed sections. Practicing composed pieces would make them better all-around. When Trey hits some sour notes and/or drops an entire composed section people do notice, it impacts the performance negatively.

No composed section of a tune is going to be a "tour highlight" but a botched composed piece could be a "tour lowlight." A poorly played Fluffhead is a good 15 minutes of a set thrown-away.

That said this FTW practice will have Trey working on a variety of chops, woodshedding hard on things that he has not worked on for a while. Getting into the mind of another player and teaching yourself how to think like they do. The comment about learning a riff from The Wheel and then applying it up and down the neck for instance. I guess the OP's point was practicing the Horse (for example, or the ascending riff in Theme or [insert part of a song you saw get botched last summer here]) up and down the neck could similarly improve his chops but also impact Phish's music directly.

I do think FTW practice will impact his improvisation more so than practicing any phish music could, and am interested to see the impact this has on summer tour.