So, here's an idea: mobs that are able to vent-crawl should also be able to access the pipes through open/unconnected/"broken" pipes, not just through vents/scrubbers. Additionally, when a mob that's ventcrawling runs into a broken section and leaves the pipes, it also removes the tile on top of that broken section of pipe, if any.
Why?
It makes sense that a mob that can fit inside the pipes can also enter through an open pipe.
It nerfs the "strategy" (which I personally think is borderline metagame) of unwrenching pipes all around the station to fuck with xenos/terrors/swarmers/etc. It will be still annoying for them if they randomly walk into a broken pipe and pop out, especially in populated areas, but now they can climb back into the pipes instead of being basically fucked/stuck in some cases, especially considering vents are welded 90% of the shifts anyway.
The tile on top of a pipe with an open end should pop off whenever a mob crawls out of it, to allow the mob to see and access the pipe it walked out of.
Might give that unused pipe cap noone ever bothers with *some* usage. Unlikely but hey.