The logic of Socratic and Legendary badges is similar:
- Socratic is given for 100 good asking days
- Legendary is given for 150 good rep-harvesting days
- For consistency with Socratic
- To give long-time users some (not too frequent) milestones to pass: even when earning 200 every day, they would not get it more than once in five months. Having one Legendary badge is great, but having 10 means yet another level.
(Jon Skeet would have 14 Legendary badges at this time.)
There may be some slippery slope concerns: would multiple awards be extended to all gold badges? But the case is stronger for Legendary than for Marshal, Copy Editor or Fanatic. The race to get most Marshal badges would encourage more query-powered flag farming, which is a burden on moderators. Copy-editing race could be similarly disruptive. And giving multiple gold badges just for showing up (Fanatic) would make the gold badge count too easy to inflate.
Legendary is not a cheap badge, and one can't speed it up by being disruptively hyperactive for a week: it rewards a steady stream of strong contributions.