Whilst it's true that sapping is unrealistically fast in the battle map, it was a genuine historic strategy. I guess the problem is the disparity in timescales between the strategic and battle maps. On the campaign map, each turn of a siege is six months. That's plenty of time to dig a tunnel under the walls and fire it. But that doesn't translate well onto the battle map.

I guess saps could be implemented in the campaign part of a siege, giving a chance of weakening or breaching walls before the battle - but then that wouldn't be as "exciting". It would be a fair bit more realistic, though. Having said that, how many real-world sieges were actually finished with saps, siege towers, rams and ladders? I've got no idea...