I agree resource buildings are very fun. Every resource gets improved by a building. If we increase the bonuses for resources, I'd rather add those effects to existing buildings instead of creating new buildings.
I like emergent gameplay where vast complexity arises from very simple concepts. Portal's a great example. We get just the portal gun, and work past a dozen or so types of objects in our environment. Incredibly deep and rewarding gameplay emerges from those simple concepts. There was a quote for a technology in Civ 4 that echoed my thoughts: "A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away."
I look at each unit, building, or policy in the game and ask myself, is this object absolutely necessary? Can we simplify things while keeping gameplay complex? The forge and armory are basically the same concept (arming weapons), in the same part of the tech tree, with the same purpose (unit buffs) and similar bonuses (stronger units vs faster units). Merging them makes the combined building more interesting.