The myth of isolated, independent sysadmins
A while back I wrote about the myth of a completely shared knowledge base across sysadmins. Well, there's a mirror inverse of this myth: the equally persistent idea that you can be a completely isolated individual sysadmin, tucked into your own silo and working purely on your own responsibilities. This is the universe in which a multi-sysadmin group has the mailer person, the DNS person, the fileserver person, and so on, and no one has to spend any time trying to bring other people up to speed on their speciality.
This is a myth because you cannot be so completely isolated unless you expect your organization to throw out all of the work that you've done when you leave. If your co-workers are so isolated from you that they have no practical ability to work on your systems, there is no one to pick up the pieces when you leave (or even when you go on vacation). Your systems will have to be replaced from scratch, or at the very least someone will have to invest roughly equivalent amounts of effort from scratch in order to understand them.
(Oh, you document things so that someone else could pick it up from where you left off? Great, have you tested that documentation, and if so, on who?)
There are probably jobs where this is the case, where all you do is write one shot programs and build one off short term systems, where everything is completely different every time and you start from scratch (such a job could be either demoralizing or exhilarating, depending). But most jobs are not like this; most places want a longer lifespan for their programs and systems, and they want more continuity. You are building elements of a larger, longer lived environment, not independent one offs.
This doesn't mean that you should try to go all of the way to even knowledge across your group, because that's still a myth too. You need to find a middle ground. My personal line in the sand is that everyone in your group should be able to do basic troubleshooting of every important system. If all other people can do when there's a problem is shrug and say 'we've got to wait until <X> is here', you've become too isolated.
I will note in passing that it is not a particularly pleasant feeling to realize, on your way out, that you have become this sort of person and that you are leaving your successor something that they will wind up razing to the ground and rebuilding from scratch. Knowing that you are going to see all of your hard work and beautiful systems thrown away, and worse, that it's your fault.
(This has happened to me and it was very educational. And I have seen it happen to other people. Because I am not exactly perfect, it will probably happen to me again.)