There's time when you expose an enemy, but enemy's turn comes first that you can't attack. Resistance Trooper' unique ability(gaining 55% TM whenever enemy is exposed) makes such case much less frequent.
When you can't expose an enemy and special abilities are in cooldown, Resistance Trooper exposing enemy with basic is the only one that can save you.
When facing pre-taunters(Baze, ShoreTrooper), you can either hope he gets killed fast enough, or simply dispel taunt with Resistance Trooper: latter the better. Similarly, when RNGesus curses you and Chirrut or Rex gives tenacity up to entire team, Resistance Trooper is the only one who got a chance, however slim, to turn the tide with his dispel.
TLDR : Resistance team can be formed without Resistance Trooper. But then, often you will regret you had Resistance Trooper in your team, cause he makes the team more stable and fail-safe.
About Arena, I can't say. Maybe GK and DN can replace Resistance Trooper, or maybe not.
