Being a former Senator, I have to assume that she was brought along for her high Persuasion bonus, in the hopes that once the Death Star was destroyed she could ask any surviving Imperials to surrender and/or join the Alliance.
Haha! But actually good point. One thing I think the old EU *and* the new movie got right is that the Empire wouldn't just fold up once the Emperor was dead and gone; there would plenty of Imperial leaders with intact military forces who would now go 'hey, *I* could be Emperor'.
-Will
Edited by wminsing