Just want to chime in and give my two cents: I really don't see the point of limiting the slots at all. Since you can only have one preparation per building per run, all the slot-limiting does is discouraging the player from trying new and untried combinations. Hell. I have not changed one single preparation since Level 1 and it's been all Level 3 for a long while. 200 gold. Why bother. That's definitely not healthy for providing varied gameplay experiences.
Now Guild Lockers. That's a whole different matter. With the new sleuth of quest reward items, it seems limited a bit. -- Fairplay since they are unlocked in the dungeon markets as well, but still, I'm a bit discouraged from going on a quest-run since it's stocked full right now.