for me, tweets are there to be actionable. Because I most @mention to companies and individuals, asking stuff. I know about cotweet and Assistly, but the problem is I can't use them on my mobile (as far as I know) but more than that I prefer desktop clients.
So currently I use Metrotwit, but I don't see any option. For me, it is all about speed and avoiding switching my current application / window! If I would like to constantly have to Tab(+Alt) around, then I'd need no solution, but it drives me crazy. Because it is already unavoidable most of the time. I want to avoid this at least for tweets.
So while I could go for a multi-monitor setup, not only do I lack the funds and time, but I'm abroad for a longer time, so all I mostly will work with, is my laptop! So this is why I'm looking for an in-built solution, even if I still have to manually process the non-replies/-mentions with NutshellMail...