Here we go into this USSJ arguement again. Lemme set this record straight:
USSJ is a boosting of power by a lot (not as much as SSJ2, otherwise one punch probobly would have disabled cell) but a decrease of speed. What makes me say it's a decrease?
1.) Goku says it is in the time chamber, when he first does it, and decides not to keep the form.
2.) After trunks is defeated by cell, vegeta calls trunks a fool and mentions to him how he saw the decrease and didn't go all the way.
3.) after he's defeated, Trunks mentions that he didn't stop training USSJ and didn't notice the decrease because he wanted to prove himself so much to his father.
4.)Cell mentions it just after the fight, that the form is too bulky for adequate speed. Cell then make his muscles huge just like trunks, but then powers back down.
5.) The biggest reason: It's simple logic... if USSJ was nothing but an improvement over SSJ, then goku and vegeta would have both used it to it's fullest extent, seeing that there is no disadvantage to it compared to SSJ. But the disadvantage is speed loss and that's why they stopped.
Trunks could hit cell in his normal SSJ form, but he was not trained enough to hurt him. USSJ was strong enough to hurt him, but not fast enough to hit him (when he actually used his speed.) Goku's SSJ1 form during the tournament could both hit and harm cell because he had essentially trained his SSJ1 to it's very maximum, whereas trunks and vegeta had not. Goku's SSJ1 was much stronger than there's. And you know goku's ssj1 cause there's no electricity or anything; SSJ2 is the level they'd all been TRYING to get to, but never reached. That's why they all freaked out when gohan transformed.