Prowl being a part of Devastator is only cool in that something-that-happened-in-one-episode kinda way... like in Scramble City cartoon where one combiner grabbed another team member to make him be a leg.
For an actual toy, it's a terrible idea (unless it's just an OPTIONAL way to combine. I'd be glad to see the functionality added in this way as a nod to IDW if all six original cons are available.)
BUT —in my opinion — it looks out of place, it's the wrong faction, Prowl has historically not been a combiner, it's contrary 30+ years of canon, and it smacks of a bad fan-fiction... drummed up deliberately to be controversial like some low-brow reality show.
(That said, as much as I've disagreed with the writing in Robots in Disguise, I've loved it in MTMTE and all the mini-series.)
Just my personal opinion. Of course I'm glad some of you like this storyline. It's a diverse multiverse, and I'm not bashing anyone. Just stating my own views and distaste at what I felt like was a weird choice that stepped on a lot of toes.