That's exactly what I saw as well. Starters that were moved to the bullpen, seemingly for good, that then made 1 or 2 spot starts before going back to the bullpen. If there was a long term opening in the rotation it was filled by either trades, FA signings or prospects in both organizations.
I would guess it is pretty hard on an arm to swap back to the rotation after an extended stay in the bullpen. Especially if that is a veteran arm that can't go to the minors to build up arm strength prior to being inserted as a starter.
Dylan Bundy spent the first half of last year in the bullpen before moving into the rotation on July 17. The Orioles did it on purpose to limit his innings.
David Phelps was the primary set up guy for the Marlins before he was moved into the rotation in August out of necessity. He performed well in both roles. I'm pretty sure the Marlins strongly considered Phelps in the rotation in 2017 because he did so well as a starter but... in the end... they like the idea of an Andrew Miller type guy in the bullpen and they now see Phelps as that type of guy.
Danny Duffy started in the bullpen and was moved to the rotation in late May. Duffy ended up being the top Pitcher for the Royals by far and one of the best starters in the American league period.
Mike Montgomery was in the bullpen and performing real well for the Mariners when they asked him to join the rotation July 10th. He made two starts for the Mariners and was traded to the Cubs and he went back into the bullpen for a full month and 7 appearances and then tossed back into the rotation for 5 starts. Montgomery did well in both roles. I'm willing to bet... that Montgomery ends up back in the rotation in 2017 after Anderson gets his 400th injury.
Chris Devenski bounced around a little for the Astros. I'm sure he will start in the bullpen in 2017 and will be called on to start at some point and he has the talent to remain a starter afterwards.
The Indians have talent in that rotation but not a ton of depth... I'd be willing to bet they'd move Zach McAllister back into the rotation if necessity requires it and McAllister has the talent to stick.
I heard the Angels GM say they signed Petit for the bullpen because he can be moved into the rotation if needed.
Adam Warren could easily find his way into the Yankees rotation with the demise of one starter.
Watch the Dodgers this year as they cleverly use the DL and bullpen to keep starters off waivers and hold depth.
Is the argument
That it never happens?
That it never works?
That the pitchers will be broken soon after?
Just because there are only a few examples every year... it doesn't mean that it's a bad idea.