First of all, I voted against. To elaborate, I voted against in a poll that appeared to be suggesting invaders who are capable of digging permanent tunnels that act in exactly the same way as those dug by a dwarf, with no prerequisite additions to the game being mentioned.
I do realize that at some future date, we may be able to build unsmoothed walls, hide revealed tiles, or otherwise repair the scar left by the suggested invaders. If a similar poll should be made at such a date, I may very well vote in favor. But until then, I oppose it.
Alternatively, if the suggestion included details that made it a bit more appropriate to the current game, I would probably support it. Most of the alternate suggestions I've heard have been reasonable. Conditions as to when it should be implemented, or with what other features, would also be enough make me reconsider.
Finally, what might be closest to this topic's intent, is a poll in which it is explicitly stated that we are voting in favor of or against the possibility of implementing this feature, or some variation thereof, some time in the future, at which point additional changes to the game may or may not have been made. This would require a third poll option however, something like 'That depends'. Of course, such an option would render the entire poll pointless, since the majority of people would choose it. I certainly would.
On an entirely different note, I get the feeling that my previous writing was a bit overly hostile. I apologize if that's the case, but since I voted based on an idea that I dislike, it's a bit frustrating to see that a lot of people apparently voted in favor despite agreeing with me, justifying it with ideas that are totally unrelated to the question.
Of course, it's nice to see alternative ideas, especially since those ideas are almost always better than the original, but there's the unfortunate fact that it's a poll... And my side is losing. What's worse is that it's in the suggestions forum, so I feel as if there's some danger of the suggestion being implemented based off votes cast by people who actually oppose the basic idea. I don't imagine that's very likely, for various reasons, but it's an instinct of sort I suppose.
As for what I DO support... First of all, siege equipment. Nothing particularly fancy; battering rams for destroying doors, floodgates, drawbridges and such, ladders that borrow ramp code to climb over walls, some sort of makeshift bridge to traverse moats... Basically tactics that counter our primary methods of defense instead of bypassing them.
After those are implemented, and supposing they still aren't sufficient, then I can understand adding enemies capable of digging. Within limits, I would hope. It's not like sieges in the real world were routinely ended by digging through a hundred meters of bedrock to pop up behind the enemy's walls.
And there should definitely be some way of removing the tunnels without collapsing half the mountain, whether that's by constructed walls, a 'collapse' event triggered by the end of the siege, or the destruction of CPU constructed pillars doesn't much matter to me, so long as there's absolutely no evidence that the tunnel was ever there.
Really though, the most important thing is the ability to defend against it without spending fifteen years creating a complete magma moat that englobes the entire bottom half of your fort. Retaining current indestructible walls should be sufficient, though I suppose there are other ways of doing it that would probably work out better in the end.