I assume it's unpopular as a tactic for the same reason "clerics" are supposedly unpopular in D&D...people generally want to be in the "spotlight" and not in a supporting role. Certainly mileage varies a lot from player to player & character to character. I use Assist Other sometimes, but only against foes our table is having a hard time hitting. Last time I used it was against some crazy/scary undead assassins...thanks Henry.
But usually my main PC casts a buff spell first thing and then futzes bad guys or "wades in."Given that PCs in Arcanis are a lot less pigeonholed into a single "role" in combat I also tend to find that there is less emphasis on elaborate group tactics beyond focusing fire. It's not "this one thing" is what i do super well, and if my thing doesn't work against these creatures then all I can really do is help a team mate "whose one thing" is like Kryptonite to them.
Finally I'd note that they aren't really obvious options that are prominently displayed. I love A:RPG but there are all sorts of small fiddley rules half buried in different sections. A lot of players may never read on a first or second pass through the rules.