In Resurrection F, the entire reason Frieza was beat was because he couldn't maintain his transformation.

That said, Akira Toriyama is a hack and his writing his sub-par. Transformations are purely a plot device and power levels were Toriyama's means of making fights bigger and more exciting. It was cheap, but I loved every screaming moment of it.

I could see where the gimmick gets a bad rap. Transformation can still be pretty fun to use though. For instance, if it's a temporary power-up (for instance Kaioken) or just a change in power set (Like the various forms for Kamen Riders) and not an asspull as you guys seem to like to make it out to be.