Having an action rollover log to facilitate an undo button adds a lot of complexity. I've only ever seen it in 4x games. Any tactical combat examples?
If you have in-combat saves, you can just save to memory or a temp file every turn, then reload to undo.
You may as well implement saves during combat for testing, anyway.
Personally, I like fairly long battles and it's nice if I don't have to finish them in one sitting. If I have an hour to kill before an appointment, I'm not gonna play a game where I have to choose between being late, rushing through it, or losing an hour's progress. Games also have a tendency to crash or get corrupted during big battles, so I like to have a few savegames during and before combat.
If people want to savescum, who cares. I don't even like to do it. If I make one dumb mistake I'll reload in combat. If that fails I'll start over from the beginning or look for a way to avoid the battle.