I think some people bristle at the thought of players "grinding" old modules to "level up" their backup character(s) (like in an on-line computer game).
And this is happening. I was judging an LFR adventure recently and discovered that 5 of the 6 players at the table were replaying the adventure - and not just for the first time. One had played the adventure with 5 other characters. The sense of mystery and adventure was lost. In short, they were grinding the adventure for XP and treasure. I'm being farmed as a judge. Add to this that they knew the adventure better than I did (my first time running it and they had been through it 5 times) and had no problems correcting my running of the event, it was a humiliating and unnerving experience.
I recommend against allowing replays carte blanche. I saw another suggestion on the board to allow replays on certain adventures. That could be a workable solution. You'll have to pick your adventures very carefully for that to work though. Either the adventure has very flexible encounters or doesn't rely on mystery or puzzles.