Interesting, I was just wondering this last night. The reason for me is to enable making the holes in the fuselage sides for wiring and pitot/AoA line entry, mainly the former. My wiring conduit will be PEX tubing, and my plan is to have the "conduit" simply project into the fuselage a bit.
The "easy" way to do this would be to hang the wings with the bottom skins off, route the conduit temporarily with the inboard ends butted against the fuse, and mark the outline of the penetration. This is a lot more precise and foolproof than trying to measure the wings and find the matching point on the fuse, but of course the hard part is needing to actually hang the wings to do it. (hence putting "easy" in quotes)
The concern about drilling the rear spar, and having the bottom skin riveting changing things seems valid. And I'm not entirely sure I want to hang the wings more times than I have to...