Though I could definitely applaud the 'excuse for a new bike' argument, unless you're planning to break commuter speed records every time you ride to work an upgraded bike will serve you no advantage. I think the terrain could force you to a change, but not likely if your route is like most commuter routes. However, if you intend to change up you're entire way of biking life then an upgrade would be worth thinking about. IMO.
My story: I commute some (15 miles one way). I ride some. And I used to go to a monthly local 10 mile time trial. I once asked someone that knew how much and the kind of riding I did if I switched from aluminum to carbon how much of a difference would it make in my time trial speeds. The answer was none. Basically, he was talking me off the cliff of a $3k+ investment he knew I would never get the full advantage and value out of. I still ride my aluminum for commutes and weekend warrior rides and I haven't been to the time trial in over 2 years. He knew what he was talking about.
Simply put: In my life and budget, unless I'm making a huge change say going from road to mountain, don't upgrade till after I've proven the need. Not before.