I was wondering the same thing. Everyone who has lots of alts seems to have really high legacy. But I even noticed myself while doing alts, I'm really not making any progress.
Based on this thread, it seems to me that a few lvl 20s wont get you anywhere. I'm guessing lvls 25/30+ is where it will start to show benefits. You have to put more work into the character to see benefits from alts.
OK - now I am confuddled (or befused).....
Checked on all my various characters, and the target legacy for 19th level is 245,000 on all of them - no matter what level they are. The 50th's definately get more legacy XP. But those with more alts seem to get more levels - does the amount of legacy XP vary with the number of characters that you have maybe?
I now have no idea what's going on (unless some multiplier is added....)