I use adium and quicksilver and growl, but don't have them in the menubar (no point in having them, in my opinion).
I see this comment from a lot of people, so I guess a retort is needed to point something out that may not be apparent until you start customizing Adium:
You can customize Adium such that it has no options bar on top of the buddy list. For an example, see my full-res screenshot
here. It's on the far left. I've blurred out the names just for privacy issues, but you can still see that there's no bar at the top for various options, such as choosing away message.
Now, one might say "hey, just right-click the dock icon and do that!" To those nay-sayers, I say.... "nay!" While that's an option, you have to go through an extra branch in the menu to get to the global status options. And even then, there's no individual options for different accounts (at any one time I'm logged into AIM, Yahoo!, MSN, ICQ, and Jabber). Sometimes I like individual states for the accounts.
Simply put- when you have multiple accounts and have the buddy list in a minimalist view, the menubar icon is actually quite useful (moreso than the dock icon).
Now, for Quicksilver... I agree, both the dock and menubar icon are unnecessary- I still leave the dock icon just for the heck of it though