I think it's a wonderful idea, and we do basically the same thing in our much smaller office (just one vase on the center table). When they're looking stale, we bring guests or interns to the flower shop to pick out the next arrangement.

edit: We go to A New Leaf in the Monadnock building a couple of blocks away from the office. Always a nice selection, changes frequently, and we can get a few beautiful stems for 5 or 6 dollars.