In my opinion, Steam is like a billboard at your workplace. Since you're always looking at it when you walk in the door, anything put up there is potentially very visible. Moreover, it's visible to all of your co-workers who are also walking in the door. It's not as visible for those who don't work there, however.
In other words, Steam has the potential to increase awareness for the title and publisher amongst Steam users, but not really for people who aren't Steam users. Steam also has a high noise ratio. They have a lot of titles that compete for that billboard space. People might find the game or publisher with a search, but if they don't search the billboard value of Steam goes down a bit. Moreover, if Steam requires some sort of exclusivity agreement for a title to be carried, it pretty much restricts the title to Steam's billboard, and thus customer base. External channels(billboards in other workplaces) are closed off unless they are Steam's billboards which you likely have little control over(unless you pay them, presumably).
Steam does have a large customer base, a percentage of which might buy the game. Then again they might not, but any agreement with Steam regarding distribution and advertisement would still tie the hands of Matrix Games(assuming they aren't already tied by some other agreement). Steam isn't necessarily the Wiz-Bang magic pill that makes a game explode with success. Steam comes with its own baggage and needs to be carefully considered as such.