I think the best course of action is to use marketing information based on the results of opinions about certain mechanics within the game, not the entire direction of the game itself. An example would be making a small crafting change which could upset or improve how crafting is perceived, such as making the UI easier, or making it more intuitive/less of a grind.
I'm not entirely sure if Marketing would be useful in the very beginning, as targeting a crowd using specific features based on results from now will be different from a year or more later when the industry standards or preferences have changed. It is certainly useful to see what the Developers should focus on, but I believe it would be something to keep in mind as opposed to a scripture one must have and then follow.. They may not realize something is clearly broken or being abused, but reports or opinions taken from the community can bring this to light. It is thus certainly helpful, but not entirely required until at least halfway into development.
This is why I would prefer to aim towards an audience, and cater to them, as opposed to catering to the market. I would still keep information on parts of my game, and use that information to change certain things that are receiving negative attention, or alter mechanics which may seem pointless or repetitive.
An example would be the current Dungeon Crawler I am writing up. It utilizes a crafting system where players defeat an enemy, which spawns a crafting table and materials as loot. The Crafting Table adds its own modifiers to the end result of crafting. One table might add '+10% Weapon Damage' to the result of what you craft with it. Some may be able to take more materials than others. Using this system I basically added Min-Maxing in a light way. However, I am conscious about having the Crafting Table be an interaction or an item. Would I force them to run by a mediocre crafting table with poor modifiers? Or would I allow them to pick it up and save it for later in case they need it. This decision could be influenced by users via polls, which would tell me what the users wanted out of this mechanic. It may even bring in new observations, the idea might be too redundant or too much of a hassle. I may have to remove the mechanic altogether to satisfy the user.