Oh my.......... this thread is really something. Catching up on the whole thing would take me a few days so as a newcomer I'll just give my own personal what others have said over the last 5 years be damned, well that is kinda the point of a thread like this anyway.
Being pretty up to date on Salvatore's Drizzt books, I for one can say that NOT having enemy sappers would quite simply be a grave disservice to the genre. The battle to reclaim Mithral hall, the fighting with the Drow and the invasion of King Oubald(sp?) have greatly colored my own concept of what a Dwarf is and how it should behave. (The goblin juicer, boar riders, battle ragers, ceiling drops,{already in game to a degree} and the story of how a Gnome helped use natural gas to destroy a portion of a mountain range are definite highlights.) Having the caverns turn into a full blow Underdark type realm would create dynamics that would really get any TSR fans blood pumping, heart racing, and mouth salivating.
The reasoning for not having tunneling enemies that I've seen is laughable to the extreme. This is a sand box and we should have a high degree of latitude with it which should include whether or not there are sappers, infiltrators, assassins, slavery and even whether unsavory things like serial killers and rapists appear in the game. From what I've seen in my metamorphic and igneous over complicated newb experiences so far, not to mention my time reading these forums, Dwarf Fortress is supposed to be the complete fantasy package and that alone means that comically inept Goblin Sappers are an absolute must.
I apologize if this post seems disjointed, or the grammar is bad it's just how my mind works. In short the more options the better as long as the game works within it's own confines. Personally, I'd like to see Toady push things has hard as he damned well please. Game design as become to much about great graphics and sound, set pieces, and other things that just cover up how much it's all stagnated. If Toady want's tunneling he can have tunneling. If Toady want's cute Christmas critters who brutally rape and mrder people, well, he can have that to. I just like bashing my head on the Adamatine-Unobtainiun blend wall that is Dwarf fortress, hell I've been ignoring Dead Space 3, Metal Gear Rising: Revengence, and Gravity Rush for the last couple weeks, exclusively playing this game that I can't get any where with. I say that more isn't just more, more is better and as long as they do it right, all features golden. That includes giant piles of excrement trying to kill me.