I started dancing shag with grippy shoes and have since switched to slick shoes. I prefer slick shoes for my dance style and because at fast feet festivals, there’s usually a fair bit of Balboa and I like to dance both. There are a lot of good things about grippy shoes and you can see some dancers dance really well in them.
In general grippy shoes make hard cuts or redirects very easy. Similar to a tennis player cutting across court.
Sliding shoes make gliding and some turns a much easier.
Take a look at this 2018 Camp Hollywood Shag Final video. For their spotlight, @Cherry Moreno and @Filip Górski were wearing (what I think are) Toms and Keds respectively. Neither of those shoes are typically that grippy but they certainly are more grippy than leather bottoms.
If you compare Cherry and Filip’s spotlight to @Julie Roman and @Sean Carter their moves have a lot more changes of direction and large shapes, this is much harder to do when shoes don’t grip. Julie and Sean do more slidy/coasting movements, which are easier to do in slicker shoes.
At this point, 2026-04-16 whether or not these shoes were or were not slick has been unverified, must ask next time I see them!