Two reasons for an artificial slot system:
1: UI is easier to design, implement, and use if the max number of buildings is a) known b) a handful or less
That is why I want a slot system AND why I want a per building/Imperial slot system as opposed to a per planet slot system.
Looking at the possible ways of limiting buildings
1. Expense... This doesn't work because what was once a limiting cost goes away with improving technology
2. Planetary Caps...This doesn't work because what was once a limiting cost goes away with increasing Imperial size
3. Imperial Caps..works because it automatically does so (there are only a limited number to deal with because the game says so)
4. Special situations required...has the same problem as planetary caps, but less pronounced depending on how rare the special is.
5. Bad side effects... Depends on the area of effect, if the building's negative effects are planetary, then it acts like a planetary per building cap. If the building's negative effects are Imperial, then it acts like an Imperial Cap. But in both cases it is more complicated than a simple Cap, because it requires situational analysis.. Much more complicated in the case of a planet because it requires a planet by planet analysis.
BTW when I said 100 to 200 I meant of All types of Buildings (ie 3 allowed of building 1, +1 allowed of building 2, + 1 allowed of building 3, + 1 allowed of building 4=6 for all buildings throughout the entire tech tree ie Civ 3 actually lets you build somewhere between 20 and 50 Wonders)
Anything that you build on more than ~5% of your planets should be like infrastructure, automatically built on planets that have some type of a generic designation. (like Focus or Military Defense level or Internal Security level)
I think one of the ideas behind having the buildings with a limit of 2 per planet is to make the planets special.
I think that is a bad idea, when you have 100 planets, they should not All be special in some unique way. The vast majority should only be part of a group ie Developed Industrial Worlds, with maybe 12 to 20 truly Special planets (the planets that actually Have buildings).