At the risk of repeating what has already been said, I think most people are here just because they're interested in the topic, and enjoy arguing about it. This in itself seems sufficient reason, really.
I also do learn a lot posting here. I don't think you really grasp how superficial your understanding of a topic is until you've had an argument with someone about it. Talking here prompts me to go and learn details I probably never would have got round to looking at by myself. Thanks to slevesque, I spent the morning learning about the fossil record of theropods; and have set myself the next text of grasping the details of feather evolution.
As for wasted man hours, I'm usually posting from work, so I see this as me getting paid to read articles about biology - that's not wasted time.
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