Forums such as this only succeed if they have a critical mass of members who are reasonably altruistic in that they are willing to share their experience and ideas with the sole aim of helping others, with no personal gain involved.

This can be diluted in at least two ways -

1) Trade members only concerned with touting their own wares (I'm not saying this happens here)

2) Opinionated members only concerned with pushing their own viewpoint without considering whether this is helpful to others - easily identified as they are usually rude and arrogant.

Both of these can be addressed by rules and moderation. 

My personal opinion is that 2) above is more detrimental than traders touting their wares as it is obvious (unless someone is extremely naive) that a trader has a conflict of interest when giving advice.  Most people take this into account when receiving recommendations from traders.

I feel that the decline in the forum is more related to rude and arrogant behaviour than traders.  However, in the name of having an open discussion, this behaviour has been tolerated which probably should have been nipped in the bud a lot earlier.