For that matter, the Nebula's very existence bugs me. It literally has all the exact same parts as the Galaxy, just re-arranged. Why would they even do that? It can't be cost effective and it doesn't hold any aesthetic value. You could literally build a Galaxy class ship for the exact same cost as a Nebula (probably a little less) and not have to devote any extra resources to R&D to reconfigure everything.
You're making rather a large assumption there. Yes, the primary hull and the warp nacelles appear to be identical, but can we say the same for the interior components? Is the warp core as efficient? Are the sensors as effective? Are the engines as powerful?
It may be that the Nebula class fits tried and tested equipment within it's hull, whilst the Galaxy is, for it's time, cutting edge. After all, the Galaxy is designed for long range missions into unexplored territory, possibly away from Federation space for years at a time. It has to be entirely self sufficient. The Nebula does not appear to have been used for such long term missions. A ship that can regularly visit a Starbase for routine maintenance and resupply needn't be built to the same exacting standards as one that has to look out for itself.
I can also imagine that construction of the Galaxy class hull would require specialist facilities. If so, then it would actually be economical to use those facilities to construct parts for Nebula class ships. Better than letting them sit idle once the limited run of Galaxys was done.