It would seem that, contrary to how it might have looked at the time, that Pumba convincing Timon to help rescue Simba actually DID save their lives down the line.
If they'd left Simba to die, Nala, who didn't even know Simba was alive, so him being not alive wouldn't change anything, would have come across both of them later on and they wouldn't have Simba to jump in the way and fight her.
I think that Nala would have killed them had Simba not been there.