Ratchet was a decepticon agent!

[Toph]

Ratchet was the most ineffectual medic ever.
He couldn't fix Jazz. He couldn't even fix Bumblebee (something that Cade and TWO little girls could do).
I don't think Ratchet had ever been shown actually repairing any transformer, in any of the movies.

Therefore, Ratchet was secretly a decepticon agent sabotaging the autobots by keeping them damaged!

[Grand Moff Muffin]

Must be to balance out the Marvel comic, where he single-handedly defeated Megatron three times.

Martin