Personally, I think it's a grand idea.
When I first saw sieges, and I closed my gates, I was disappointed to find there were no Siege Ladders, or diggers. Yes, I realize there more than likely will be something of this sort in the future, and I'd still like to see it.
Firstly, it'd make it more challenging. Sure, noone wants enemy forces digging their way into their fort, but sometimes bad things happen. I'm not saying all sieges should have this, but a huge siege with little way in would be stupid break against an enemies walls over and over until their forces are destroyed. I build my gatehouse explicitly for the purpose of slaughtering as many goblins as is dwarfenly possible, with as little casualties as possible. I can't be the only one here to do that. It can be satisfying not to lose to a surprise attack and a sudden upset of the balance of war, but it's not necessarily challenging or entirely fun.
It's been mentioned before that perhaps mining needs a little reworking, that it goes by a little too fast. A major siege is full of soldiers, not expert miners. Sure, an engineering team may in fact have miners, but as we all know: Dwarves are the most superior miners there are, and a Legendary Dwarf Miner more than doubly so.
It should, by all means, take other races longer to mine a chunk of earth out than it would for a dwarf of equal skill. Sure, it would take longer to dig out. But if you have an enemy camp holding back, you're going to begin wondering what they're doing, aren't you? They aren't going to start digging within crossbow-range (unless they're stupid). They'll start digging far out of your range, which would make digging even harder because of increased distance. At this point, you're going to wonder where they went and go looking, probably to see some ramps or stairs connecting to a tunnel below them. This would be a big "Ohnos! time to do something!".
I think this would sieges all the more real. However, I admit, making it so you could remove tags so this doesn't happen wouldn't be a horrible idea, since it would be a compromise between two schools of thought.
I also would personally like to see siege ladders and perhaps siege towers occuring more often than attempts at digging. Sieges always ending up with digging would get tiring. Digging is risky business, especially in soil (which would be far easier to dig long distances through than rock). Digging can be discovered by enemy forces, and dealt with. Siege towers and siege ladders have their perks, and are probably easier to make on-the-fly (assuming your map has trees) and a lot simpler to use. I can't see small sieges really using anything other than these. These are basic tactics. Smaller, earlier groups would use these first, while larger groups of multiple squads with local leaders might use multiple tactics, including advanced tactics of siege equipment, siege weapons, and digging/"sapping".
How could AI/pathfinding for digging work? Well, you don't want these guys digging all the way across the map, that'd take forever and would end up being annoying. They'd want to find something relatively close to dig through in hopes of finding your fort. If you plan accordingly, such as digging a fortification tunnel (a square tunnel around your fort that acts as a debuffer (It would simply cause them to run through the tunnels you've planned out for them to folllow, looking for your civilians, and a place to ambush you, while being lured into another trap unbeknownst to them).
Whats to stop them? Well, chances are they'd think they've found something, and follow it, looking to jump someone, and not continue to dig. This would probably be catastrophic for them: assuming they've found a good path. If they end up being turned away (forced to flee, but not off the map?), they could be reset to attempt to dig again (if any of that initial party survive). Lay plenty of traps in these tunnels. It'd take some extra time, but extra time is worth extra protection, isn't it?
I think this would add a lot to the game, rather than take away from anything.