1) easy way to earn legacy points
2) new story arch
3) more lvl 50 abilities to choose from when legacy goes live
I can think of a ton of reasons to roll alts.
Leveling alts does help. I am sitting on 14 leg points now running a lvl 50/ 45 PvP rank, 30 Gunslinger 10 PvP rank and a 27 Vanguard. Leveling the alts is a great way to make Legacy points. But not the only way, no.