First of all, "Overkill" does not exist, there is only "Just Enough Kill"

Beyond that, if you want to spread things out across multiple screens, go for it. I use 2 21" screens at work and love it, I could not imagine not having them at work. at home, different story, different functions. My MacMini is hooked up to a 65" HDTV - dont really think I need to go dual screens, however it is strictly a media server. My MBP is my main machine, and it is generally out in the living room where I have the big TV and i am doing most of my computing. When it comes to getting into FCP and doing some work, I prefer to be elsewhere, so I pack up the MBP and go - no dual screens. If I want to move something on my MBP to a different screen, I use AirDisplay and my iPad.