Do you think have having a site forum under /forums from your main domain can ruin your sites key words and if its removed your keyword quality will increase?
Is the topic of the forum related to the topic of the site? A forum can help your site as long as it's on-target and used.
Not neccessarily at all. Removing pages or restricting from robots.txt can increase your SERP definately. Indexing every page isnt always a good thing.
You should really be asking, does a forum adds to the overall value of your site in your readers mind?
No you shouldnt. This is SEO not RMO (readers mind optimization). I dont mean to be at all patronizing, to anyone specific or in general, but - it may be my fault for not being more clear in my original post, but the quality of posts has been wayyyyy off topic. I shall make sure to cut any ambiguity in future.
Not at all, forums are very powerful for seo I use them and rank all over the place and rack in traffic and such!