Replays often glitch and crash, the editing you can do with them is limited(aka no fancy smooths), they have no HUD elements whatsoever and overall it's just a bad counterpart to demos except maybe that editing replays is easier to get into + you have a 3rd person camera available(that you can turn freely) in case you don't have access to a STV demo.
Then again, if it wont have any negative impact on server performance then by all means, go for it.