I think they would increase their market-share exponentially, especially if they released a custom ROM SDK or the like.
I agree and disagree. People who flash roms are probably less than 1% of the market, which is only 900k globally (assuming 90M Android sales this year). That's nothing to sneeze at, but at least a few phones are/will be open and even then there's no guarantee of capturing all or even most of that 900k, so the incentive simply isn't there.
However, I would agree a savvy company might embrace the rom community and work with developers a bit more closely and they could really up customer service/satisfaction while reducing maintenance costs by pushing some of that off on developers. You make it a very seamless and easy process to flash roms (read: like installing any app from the market) and then you maybe have a strong selling point.
On the other hand, with Google planning to slow down updates to 2 a year it becomes less critical. But I still think it would be in the OEM's interest to take VZW out of the loop, which only serves to add another layer of complexity to the update process.
Root adds some value, and most of these phones can be and are rooted. But to be honest, I don't see a lot of value-add from custom roms, beyond getting the latest version of Android and propping up the underpowered D1 (which is mostly a non-issue with what news phones have under the hood).