People should not be able to access data they don't need. If they need it, you can grant it. But the assumption should be that someone who doesn't use the design server shouldn't have access to the design server.
I don't want to know about or see information I don't need to have at work. It's not that I'm not trustworthy. It's not that I would abuse it. I just don't need the liability that it gets out through me.
Because your account credentials could get stolen. Your laptop could get stolen. Your laptop could get hacked into. Your laptop could get malware. Reducing the list of people who have access to a resource insulates against all of these things... automatically. And sure, all of those risks have other ways to mitigate them as well. But layers of security is key. And hey, it also stops employees from sneaking off with your data too.