Vote YES! Because it'd be sweet.
I do agree however that it'd be a pathfinding and aesthetic nightmare. So why not make it invisible?
Like. The tunnels show only for the animals tunneling and that's that. Dwarves can't use them even after you've noticed they're pouring into the fort ... and the only way you notice the entrance into your fort is because an engraved wall has become regular rock ... and antmen are eating the young. Whichever you aesthetic types mind the most.
What do you mean, make it invisible?
Do you mean "making it so the player can't see the tunnels"? If so, that
doesn't change anything. It still has to exist, it still has to be made whether or not the player sees it. It's still going to be a giant clusterfuck of a mess and it wouldn't fix any pathfinding issues. The game still needs to keep track of it, so what's the point of making it invisible? So it looks prettier? Why not actually make it so that doesn't happen, and take care of the problem rather than ignore it?
And not all of us are talking strictly "antmen" here, we're also talking about actual enemy civs sieging and tunneling.
And no, if tunneling was coded correctly, which i'm sure Toady is more than capable of (considering all the things he's done and is currently working on) doing so, this won't be an issue. All you need to do is limit the actual activity of such digging units to a certain area. Whenever they begin digging, they'll dig down and using some random pathing dig until they get a nice "home" going, and from there they can only dig so much and so far. As long as it's coded that nests don't grow to make up the entire map, and stay somewhat self-contained, we're fine. I mean, a few antmen are fine. But even if they breed like rabbits, it'll take a dozen (or more) years for them to grow so large that they'd need to expand their nest to massive proportions that would become truly inconvenient.
Oh and wait, if they're too close, or you don't want the potential nuisance, then why don't you just exterminate them all? Creatures are a simple problem. Sieging enemies pose a greater challenge as they would intentionally be trying to dig into your fort. Pathing could be a problem, but as long as it's coded that tunnels are to be relatively straight and 1 or 2 tiles wide, we don't have any issues with swiss-cheese.
If a tunnel exists, why would you bother digging a whole brand new one? Just reuse the same one and dig on through again. Though i'll admit, over hundreds of years, you might end up with a few different tunnels into your fort, but by that large amount of time you'll have expanded out into most of them and covered them up and used them for yourself.
If done properly, there is no aesthetic nightmare and no horrible pathfinding issues.