Sigh, I guess nobody bothered to read my wall of text, it's my own fault, stupid spoiler...
Tunnellers would have pathfinding, the tunnels would not be random, and they wouldn't bother if there was already a tunnel to the place they wanted to go. So the swiss cheese analogy shouldn't be relevant except in the early testing phase, which, given that the game is still in alpha stage is likely to be a while...
Being able to engrave walls is only a minor detail, in-game it is a bigger issue that they will dig through masterwork engravings than that they will stop you from replacing them... To be honest, I have no idea why walls can't be engraved, all I can think of is that it stops people from dismantling engravings and rebuilding them until they have one they like, you could stop this by putting the engraving on the block that the wall is made out of, but then people could just fill a room with all masterwork engraved blocks, but that would be a cool way to make libraries, you can have the notable dwarves section, the ancient history section, the demon section, the history of trade section... Perhaps engraving on blocks could have their quality reduced every time they are dismantled...
I would really like invaders to get tired after digging out a small number of tiles and then have to go back the the siege camp to rest, if there isn't a siege camp they take their pay check and leave... Mining could be cool if it was done on a squad by squad basis, each squad decides how it is going to attack, build their equipment, wait for the signal, and then attack from a several different fronts all at once, of course, sometimes 100+ goblins will all flood your main entryway to see if you have been spread out too thin...
Tunnelling is unique in that it is the only way that enemies will be able to bypass the single tunnel in the ground entrance. No amount of pulling down drawbridges, catapulting constructed walls, tearing down constructions or building over channels will have any effect at all on my current fortress, which seems to be pretty much immune to goblins with all of a couple of dozen whips in traps and a couple of dozen stonefall traps spread out over a 3 wide corridor. Even without the traps they still need to deal with a completely underground, 4 storey tall archer tower and a catapult range that is probably about 40 tiles long before actually getting to any civilian paths. So long as attacks rely entirely upon the paths that the dwarves make they will have no way at all of dealing with a fortified bottleneck other than trying to overwhelm it with numbers.
+ - smoothed and fortified rock
# - unmined rock
- open space
A - axedwarf
M - marksdwarf
g - goblin
gg###
##g##
M+g+M
MAAAM
MAAAM
How many goblins are required?