Originally posted by Nameless:

Other stuff blocks spawn too, like campfires

Yes but the range is very small. Not sure what exact U distance is for objects like campfires, hearths, etc but they are really small, lot smaller than even default workbench.

To cure OPs problem of fully spawning proofing mobs popping inside his base, as a practical pov, things like campfires are completely irrelevant b/c you'd need so many just to protect even a small portion of base.