Beneficial or not, just about everything he does up until the end of the cell saga is for himself.
Saiyan Saga: He's conquering a planet. He kills Nappa. The fact that he told Piccolo that he was a Namek, from the planet Namek, which leads to the Freeza Saga, wasn't "Vegeta helpin' out tha crew." He expected to kill Piccolo, as well as every other living thing on planet Earth.
Freeza Saga: He killed a lot of important henchmen-type guys, but it was all for himself. In fact, until the last few minutes of his life, his plan was "help the morons out because they have the radar, steal the wish, kill them all." He did buy time, like with the Ginyus and with Freeza. But he fought the Ginyus because he had to. He saved Gohan and Krillin because he needed their help to find the Dragonballs, and he fought Recoome because he had no choice. Same with Freeza. He couldn't run (remember he tried to?), so he fought him. Got his ass kicked too. But as a wonderful bonus that he obviously hadn't intended for (he thought he could beat Freeza; he thought he was actually a super saiyan) he bought Goku JUST enough time to heal and get there.
Cell Saga: He lightened up after all the Freeza stuff. Saved Goku! His first intentional good deed? Nah. Remember, after Goku beat him on Earth, Vegeta wanted nothing but to beat him. In fact one of the corny lines he says, which is often used in anime when a bad guy type character actually saves the hero from another bad guy type character, is that "I won't let anyone kill him but me." Fast forward to the room of spirit and time: You say if there was no Trunks, Vegeta wouldn't have learned Final Flash? Wrong. Vegeta and Trunks NEVER sparred or helped each other in the ROSAT. They were both on their own. That just shows you how much of an ass Vegeta is. Fast forward again: Vegeta lets Cell go perfect. Big, stupid, ****y mistake. After that he saves Gohan. For Goku's sake, maybe, but also maybe because if Cell won, everybody would die, including Vegeta, so if you want to look at the bottle half-empty, that's the explanation. And you can't really contest that
Buu Saga: He's a good guy now, pretty much. But...His desire to kick Goku's rear turns him into a temporary servant of evil again. The resulting fight creates Majin Buu. If he had just been a good little boy, and fought in the tournament, not enough energy would have been supplied for Buu (they agreed not to go SSJ in the tournament, or people would freak out.) Then Gohan would have been drained, they all would have gone to Babidi's, and then Dabura and the other henchmen would have been blown away. But anyway, it goes on, and Vegeta sacrifices himself. Now we know Buu is practically indestructible. Well, it's the thought that counts. After the fight with Goku, Vegeta DOES definitely fight for other people for once. So thanks for that.
Meh. He was a prick for most of the series, but pricks have their uses, even if they don't intend it. He was a badass, and everyone likes the badass, so yeah, DBZ would have been much more crappy without him. After all, bad guys are ALWAYS way cooler than the good guys in just about everyone way. Just look at Cobra and G.I Joe. Zakus and GMs, Ninjas and Steven Seagal, etc.