First, OP, there is some confusion between "requiring steam" and being "steam exclusive." Steam exclusive means you can only buy and install the game via steam online. Requiring Steam means you can get the disk at a store, but you still need steam to install and activate your game.
Some people will not buy steam exclusive games because of a fear of not having a physical disk. This can lead to some confusion in your poll.
Now then, if the game simply requires steam then I will buy it and play it. Steam doesn't crash or use any noticeable resources on my system and I have never had any problems with steam limiting access to my games. I like steam becaues I'm lazy and it makes things easy. Simple as that.
So yes, I will buy the game no matter what. I may, in fact, be one of the few that would even buy this game if it were truly steam exclusive (no disk).
Civilization 5 disks were sold in stores. One must install the Steam client app and run Steam in order to play the game. My understanding is that Fallout New Vegas is the same.
You may wish to distinguish "requiring" Steam and "Steam exclusive" based on the fact that disks for these games were sold in brick-n-mortar retailers or amazon, etc. However, I believe that is fallacious.
At the end of the day, it was not possible to play either of those games without installing the Steam client app and running Steam to make the game run. Other than Windows and/or some Windows OS related apps (Direct X?) this requirement to patronize and use a third party app only applied to Steam.
Thus, in my opinion Civ5 and FONV both require Steam and are "Steam exclusive." Even versions of the game bought via other distributors still require Steam use = use is exclusive to Steam use.
In contrast, the example of Dragon Age 2 as well as Mount&Blade Warband are good examples of games sold with "Steam Optional."
I bought my verstion of Warband on Gamersgate, a strictly digital distribution service something like Steam. I authenticated it using their serial key proces, and now play the game whenever I want, and without any of the complications of being involved in Steamworks. Other guys who, for example, prefer to use Steam, bought their version on Steam online store, and their version requires Steam to play = Steam Optional, the best option for publishers to choose based on the distribution of respondents in the surveys that have been run here and on Survey Monkey.
By making Steam ONE of at least two if not several options for distributing, activating, and authenticating a game, all segments of respondents to the surveys that have been run can be satisfied customers. Making the game require installation of the Steamworks client app and networking to Steam to activate, authenticate, and continue to use the game in an ongoing fashion will clearly alienate a significant chunk of prospective buyers who either say they would prefer there to be options, or that they WILL NOT BUY it if it requires Steam.
Debates about Steam-Love vs Steam-Hate and how Steam compares to other distribution/security providers are moot. There is a chunk of prospective buyers who want other options.
Unless the financials from going Steam exclusive are clearly and sufficiently favorable compared to those of going Steam optional, Bethesda would be wise to weigh the decision to go Steam exclusive quite carefully because these data suggest they stand to alienate and lose significant segments of prospective buyers. This fact alone is in the medium to long-term going to contribute to growth of the "No DRM!" if not "No Steam!" policy segment among game publishers who may be well-positioned to step in and take those prospective customers alienated by Bethesda and make them brand loyal customers of their own.