Vegeta had a scouter and didn't see any power levels worth a challenge. Which, speaking of, should Kaio-ken have been plenty enough to deal with Vegeta, if Goku's power was 9000 and Vegeta's was 15,000?
As far as Frieza, he never needed his extra forms. Not until Piccolo really. (I can't remember what drew him to his second form)
Cell was looking for his final form the whole time, he only wanted to be complete.