These things:



They come in both floor and wall-mounted varieties.
Seems to me, all they do is obstruct the placement of more important objects. Since any house that is weather-proof is also considered to be heated/cooled, I'm thinking of removing them from all my houses that have them. Any reason I shouldn't?

(As an aside and while I'm on the subject, I've seen houses where some rooms had them, while other rooms didn't. That's not the way it works, folks. If a house has central heating/cooling, then all rooms must have a vent (except for closets, etc.).
Similarly, I've seen houses where the builder put a single radiator somewhere, apparently as a decorative object. Again, if a house is heated this way, then all major rooms must have a radiator (including bathrooms). Just because Maxis put an item into the decorative bin, doesn't mean it should be used as such.
If you want to be realistic, you have to either include these items throughout the house, or leave them out altogether.)

I believe that they are just decorative.

Well mostly, yes.
However, put a radiator into a house, have a Sim turn it on and then warm itself. It's kind of funny, because they will point their arse toward the radiator to warm themselves.
So perhaps the radiator can serve the purpose of warming a cold Sim.

I don't think they do this with vents though.

Yes, if Seasons is installed, radiators will function and will warm up cold sims.

I don't see why a windowless room couldn't be given a vent and not the rest of the house.

I've heard that about the radiators but never tried it.