The test of a SP these days is essentially, can you reliably (>50% of the time) make it thru the first four batters of a team's lineup three times. If so, start your own games. If not, you need an opener with the hope you can give the team five innings instead of just four.
Berrios and Pineda seem fine (Pineda's pitch count can sometimes get worrisome, but seems solid for 90+ pitches each outing). Gibson I'm fine with in the long run, even if he is closer to a 4.20ERA pitcher going forward.
Odorizzi has routinely been the type of SP that cannot get the 19th batter out with any consistency...and it typically goes downhill fast. Even when he's cruising, he crumbles. He's an enigma who looks like a bonafide ace for the first 18 batters. He is the poster-boy for using an opener to squeeze an extra inning out of each day he pitches.
Pairing an opener with Odorizzi could potentially save the bullpen from 30 innings of work over the course of a season. That's huge.
We'll see how Perez performs as a SP here, but the bullpen numbers are not encouraging. While he may need an opener as well, he is not comparable to Odorizzi in why he needs an opener. Perez seems more like a 4.80 ERA guy that needs an opener due to futility.