The problem with enemy diggers is if they destroy your engraved stone you can't fix that. Constructed walls can't be engraved and "natural wall" can't be replaced. (Barring obsidian, which has its own problems.)
Hence my point that you should build a rough outer section such that diggers won't want to dig your engraved stone. Anyway, by the time we get digging enemies, chances are Toady will have figured out what he wants to do to make constructed wall engravings make sense. That's the only reason he's not doing it at present (and you know, all the pile of other stuff he wants to code) - he feels it's fair to have constructed walls be engraved but right now it doesn't make sense to him (see DF talks).
That is my point, if the player doesn’t want something why make it a situation where you throw the baby out with the bath water and they have to disable invasion just so they don't have to deal with making parts of their fort they don't want to make, the fun of DF is the right to play as you see fit, railroading players to make "anti-omnipotent enemies areas" seems more like railroading.
I’m fine with digging enemies as long as there an OPTION not to have them.
Chances are, if they don't want diggers period, then it'll be a raw modding job - just like aquifers. Ok yeah, I should have made that more explicit in my post - I automatically assume people are used to the idea of taking bits out of the raws, like knives for snatchers or maybe bows & crossbows for goblin invaders. Still, there will be people who insist on not touching the raws (or the advanced worldgen options) and among them there will still be people who don't want their Aesthetics destroyed.