Honestly, I would include marketing tasks right from the beginning. Someone with marketing sense should be looking at your Game Design Doc, to help make sure your focusing on salable features, instead of white noise. Naturally, anyone can take on that kind of task, but the more experience the better.
yes, if a project is going to be for-profit, then that affects everything: what to build, platform choice, tool choice, available graphics engines, libraries, etc, and skills in the use of those. Also, a for-profit project should continually strive to achieve "buzz" about the product, starting from the first day of conception of the game idea.
As for marketing influences on the design doc, i take that with a grain of salt. Yes, it is good to evaluate the feature set based on selling point potential. However, while an experienced marketer can say what sells well, there is often a tendency to attempt to modify designs to increase profitability at the expense of the design itself. IE make it more of a "this, that, or the other" type of game cause that sells well. If the original game type and the "sells well" game type are too dissimilar, the result is a compromise that appeals to fans of neither type. Its also possible that market driven design will overlook profitable but "less than best return" titles, or pass on the next new killer game app cause its not the current "sells best" game type. In the end games must be approached from a gamer point of view, not a marketing, coding tour-de-force, artwork, or storyline point of view. Bottom line: cool games are what sells. And a gamer is more likely to know whats cool than anyone else - including marketers and publishers. After all, gamers are in this for the "love of the game", marketers and publishers are just in it for the money.
"DirectX is like a belt fed machine gun, where every texture change is like hand loading in a new belt of ammo. worse, every mesh (vb) is a new belt of ammo, and a texture is like breaking the gun down, and setting it up again elsewhere, then loading it, then spraying triangles again. so you want to setup the gun once, string all your belts together, load it once, then just spray."
Rockland Software Productions
"Building PC games since 1988"