I think for those reasons CERN is running without a superconductor mode and can hit the API DB from the cells. Devstack superconductor mode is the ideal though for the separation of concerns Dan pointed out.
I'm not sure that CERN does run without a superconductor, but maybe. However, cells having API DB creds is a different issue -- you can give them those creds and still have a superconductor. Without a superconductor, you'd have to choose one of your cells to receive the burden of cross-cell work (like instance boots, resizes, and anything we currently don't target to a cell. If you have one big "main" cell, and a bunch of smaller ones, that could make sense as you have extra capacity in the big one. From an organization point of view, and from the deployment tool's perspective, that makes it a little more lopsided than ideal. However, again, if the overhead of running those services is too great, then it makes sense. --Dan