And I am definitly on the flipside. Without a way to fill in tunnels again, it would make a mess of things, completely destroy attempts at aesthetically designed fortresses, and finally, how would you like it if they burrowed through a bunch of adamantine, didn't drop any ore (because they're seigers) before coming into your base.
Or what if the pathfinding A.I. wasn't 100% solid and they tried burrowing through a magma pipe or an underground river?
Finally, tunneling wouldn't be that fun if it was predictable because then you could just build a "decoy" outer wall that the AI would ALWAYS pathfind to which just happens to have your legendary axe/mace/hammer/marksdwarves standing at the ready. (Similar to the "sacrificial goat on a chain" exploit now).
Where you tie up a random animal waaay outside of your base because the A.I. will pathfind towards it first and it'll pop the ambush or seige. Then you make it a really twisty passageway and it ties up the enemy seigers while your dwarves scurry to safety.
I think if we're going to include tunnelers then they should defy normal defensive tactics. The whole point is to make seiges faster, but as is the only thing you'll need to do is build an "outer gap" between the edge and your base that leads through another hallway of traps and then all that coding has been wasted on a feature that doesn't add anything to the game. By not being able to predict (or being able to predict only on a short-term notice) increases the Fun and danger. Imagine the "fun" those 30 goblins spawning BEHIND your trap corridors/archery towers/trap fields/drowning chambers?
Now, I'm not saying it should be completely random or brutal, but give it that spice of unpredictability. Make it so that they can pathfind through unmined ore across the z axis.
So they could go up and over your defensive permiters, or possibly under it all and come up through your basement. It would be possible to defend
some things, but given the vast amount of unmined rock a typical fortress has makes it a lot easier for seigers to actually do some damage.