I only ever have a problem with necrobumping when the person doesn't add anything worthwhile to the thread. If new information surfaces that is related directly to that topic, then yeah, feel free to bump it and post the information. If it's a thread related to a specific game (for example, ameratsu's mega Demon's Souls thread), and a new player is looking for help, that's fine too. On the other hand, just quoting somebody to say "I agree" or some other silly one line response should be discouraged.
Simply locking all threads is a bad idea, given there may be legitimate cause to bump a thread.
Edit:
I get the feeling many here are stuck in the mindset that bumping any old thread is bad, which I guess is something that has been ingrained in many posters' heads over the years. Instead of assuming that all bumps outside of a certain window are a bad idea, look at why it would be a bad idea. There are many reasons why resurrecting an old thread could make perfect sense.
