The Problem is warfare. I just recently ran a Wormhole only game and I noticed the issue. Let us asume 3 Empires: A, B and C.
A and C both are a peace with B. But A and C are at war.
A and C share access to B's Wormhole stations.
Either side now has a set of Wormhole stations that can not be destroyed. Destructible stations is a core balance idea behind Wormhole.
That is why we do not share access with others:
We would either create undistructible stations, wich is a balance issue for Wormhole.
Or get our stations destroyed as part of a war we are not part of.
And that is before we consider stuff like another fleet having a station in use when we wanted to use it.
This is actually something that's worth thinking about, and I think at the core the aspect of A and C being at war but using B's wormhole stations as a medium of transport is a pretty good circumstance.
Think of it this way: B willingly, voluntarily gave wormhole access to A and C. As a consequence, it allowed itself to become a vector through which warfare happens. This is actually an interesting situation that adds some real depth to this idea. Yes, those wormhole stations can't be destroyed, but that issue goes both ways: you can't stop your enemy from traveling to your space now, but they can't stop you either. This is really not very different from how hyperspace and warp travel work anyhow albeit with the consideration of where they're coming from (which was still an issue anyhow).