I'm not sure i completely understand this part. So designers are valued less because it's more difficult to be a good designer? Wouldn't it make more sense to say that something that's more difficult to excell at be valued more?
Sorry I should elaborate. I think that designers are valued very highly when they excel at their work and have the varied skill set. The current reason, in my opinion, that designers are less valued is because most beginning designers believe that their sole purpose on a project is to provide 'ideas' and tell people what to do. For that reason the best designers usually come from different backgrounds, such as programming and art. So I guess it's best to say that ONLY being a designer is less valued, because there is only so much you can contribute to a team. A game designer has always been a vague and hard to define profession.
I don't understand this part, what kind of documentation?
I think this is a pretty good example of a game design document: http://www.runawaystudios.com/articles/chris_taylor_gdd.asp
A game design document should be the 'go to' place for any development questions, as well as game concept questions. Theoretically, a programmer or artist should be able to build the game completely based on this document alone. Obviously this isn't necessarily possible as the documentation for a game is constantly evolving and changing. But you get the idea.
'extremely good' was a very vague metric, sorry about that. Basically my advice would be to build up your experience, because that will be your saving grace. If you have skills as a concept artist, then by all means use that skill to help convey ideas and to refine them as well. The most important thing, in my opinion, is to make sure that during a game's development you ALWAYS have something to do and contribute.
Let me know if anything I said didn't make sense and I'll try to clarify.