Just a few points:
- The problem with replays in the beta was that they were bugged so they would often freeze partway in the game or would desync from the original.
- Replays were playable, however - Poita's "mod" (it was really more of a separate tool) just made it so that double-clicking a Demigod replay from Windows Explorer would automatically launch it in Demigod.
- The actual ability to play replays was already in Demigod; it was a more basic version of Supcom's. You could speed up or slow down the replay just like in Supcom. They just had the "replay" portion of the game disabled, much like the "singleplayer" option was disabled.
- Rewind doesn't work (or usually doesn't, anyway) because replays simply keep track of the commands that each player made. Playing a replay just tells the game to recreate the match. This is why you can't just jump ahead in the replay; you can fast-forward, but the game still needs to make all the calculations to recreate the match.
- Replays are great for learning from the better players. They also let newer/weaker players show their games to people so they can get advice.