Today, we take presence for granted, but early on, it was the most efficient way to manage day to day interactions with co-workers. That said, people were just as likely to use it as another filter or gatekeeper to make them LESS accessible. Why? Because presence was making them TOO accessible, so by setting your status to DND, the unwanted intrusions would drop off, and then you could actually get some work done.
Well, that's just one take on presence, and however you look at it, there's a strong case to be made that it's relevance has passed. It's not going to disappear anytime soon, but a lot of things have to go right - and a lot of rules have to be universally followed to get full value from presence. Basically, people rely more on text/IM/SMS now when they want to find someone, and there's not much etiquette any more about bothering people. It's widely expected that with mobility you're always available or at less reachable, and in that case you really don't need presence.
That's the gist of what we covered on our last UCStrategies podcast, and there were strong views on both sides of the coin. It's a fascinating topic, and yet another example of how the UC space just keeps morphing along. The podcast was moderated by Phil Edholm, and I hope you give it a listen - here's the link.