This is the question of the decade. In my opinion reading forums dedicated to training or whatever is not always waist of time because you can find some useful information. However writing in forums is a huge waist of time for the following reasons.
1. Too much energy for nothing
Writing on forums takes time, energy and in most cases there is no point. In 99% of the situations the question you would ask has been already answered and you just need to search the forum instead of typing.
2. Too much arguing for nothing
Let's face it - everybody likes to argue on the Internet - especially about training. What's the point? Seriously. Huge waist of time. Step back and watch the morons fight.
3. Making money for somebody else
By writing on a forum you are essentially increasing the content of a website which is responsible for the site's traffic. And the traffic is what makes money. Why would you do that? Just start your own web-site/blog and make money for yourself. Why would you make money for somebody like Scooby, the natural liar, or bodybuilding.com or the skinny fat nerd Lyle McDonald. Think about that - start your own shop.
4. Forgetting the real world
I am not going to lie to you - I spend most of my time in front the computer - work, watching movies, writing this...etc. I hate it but I have to do it. I would not like to increase the time I spend in front of the PC by waisting time writing and arguing on forums.
This is it. Use the forums for information but stop there. There is no need to do so. There are people out there with registration on many training sites and for what?
Let the morons fight. Use them.