Red Vent- most ppl who teleport up there use chutes to get over to the corner of the roof. Unlike some players who block, when zombies are starting to get through they teleport and suicide.
Building Vent- Leads to another vent that I have seen many players use since a lot of the newer players have no clue how to get there.
Completely useless or not, your still blocking access to another area. Until it changes (Using a players body to deny players access to a certain area)