The other thing to consider is that even after Galactica
had jumped, there were still civvie ships to be protected (Colonial One didn't even leave until after the Pegasus
had been destroyed).
Had Lee not jumped in, then Galactica
would have gone down, leaving the Basestars and Raiders free to concentrate on the Vipers and civvies.
remained to fight alongside the Pegasus
, in her damaged state, a single Basestar could have remained to finish her off while the others engaged Pegasus
with the result that Lee would have been forced to jump away to avoid her own destruction, once again leading to larger civilian casualties, along with huge Viper losses as these would have needed to either be abandoned or fight their way through the Basestars and Raiders to reach the Pegasus.
took the arrival of the Pegasus
as an opportunity to get out of harm's way, fix the FTL, land the Vipers, then get the hell out of Dodge.
Lee simply took over the duties of flying target to distract the Cylons, and in the cause of saving the civilians, the Pegasus
had to be considered expendable, a fitting end that vindicated her after Cain saw civvies as expendable to continue her fight with the Cylons.