Two, Valve may offer the Steamworks DRM in exchange for an exclusive. That saves Bethesda money, time and resources in purchasing and supporting a different form of DRM. Valve handles all NV DRM authentication through Steam as part of the installation process.
This presumes that the Steam DRM actually provides a benefit to the publisher though. Ostensibly the point of the DRM is to reduce shrinkage from illegitimate copies of the game. My understanding is that Steam is no less a joke on this account than any other form of DRM.
Moreover, based on the thinking of some (e.g., quotes by Brad Wardell of Stardock) DRM such as Steamworks may actually promote piracy by creating an adversarial relationship between publishers and consumers. According to Wardell, making good games and having warm and friendly relations with the gamer community is the key. This makes sense to me. A would-be pirate or former pirate is more likely to pay a game company that they respect and admire, but more likely to want to harm a company that they find loathsome and alienating. In this respect, I suspect the "benefit" to the publisher actually works in exact reverse to the direction you are arguing, i.e., that it represents an overall reduction in profits.
Three, Steam exclusivity means that the vast majority of players will have their game automatically updated as patches are released. This cuts down on support costs for most publishers (people complaining of bugs who haven't installed the patch that fixes the bugs is a major and common problem for tech support). Steam also provides a nice simple installation method, with automatic updating of Direct X if needed which Bethesda no longer has to worry about. And Steam offers a handy little "Verify integrity of the game files" button which can eliminate a lot of problems people have by repairing the game in case of a file error (this also updates the game if necessary).
Also problematizes modding, and creates technical problems of its own by potentially inadvertently leading to patching when an end-user did not wish to do so. Also funnels user problems through Steam's support, which I understand is quite useless, and if nothing else, is just the distributor, not the actual maker. Moreover, Steam is unnecessary to derive the benefits you list, as far as I can tell. It appears that Paradox does just fine keeping their myriad patched games going without resorting to Steam's auto-patching aproach. Really what this point is about is volume, and reducing the actual level of support which a publisher risks having to engage in with end users, i.e., an increasingly corporatized model of game production in which there is more distance between end-user and maker with Steam in between.
Right now Valve has dominance. But the fact that selling through the dominant distributor may increase volume is not a good argument to NOT sell through every other distributor too. Not unless Valve is engaged in contractual practices that compensate for the costs in terms of profit share. Based on comments by Devs, I have my doubts that Valve actually does that. I've seen at least one forum poster with a Dev 'rank' say that "30% is what they all charge." So if Bethesda can get Valve to distribut FONV or Skyrim by sharing 30%, and they can get D2D, Impulse, Gamersgate, Amazon, etc., to all ALSO distribute for 30% where is the real value to Bethesda? I honestly just don't get it.
It's also possible that exclusivity means a more prominent position in Steam's store (New Vegas has been in their "hot releases" banner since before it was released), which means more sales.
If Valve is not about about promoting gaming, then the quality and popularity of the game should determine the prominence not the reverse. Again, this 'benefit' shows the sippery slope of an increasingly corporate model in which, hype and advertising begin to take over as primary determinants of sales from quality and value.