Well there is the value of the Steam Store and then the value of Steamworks though. So being tied into the later can still provide a lot of value compared to the former for some developers\publishers. Obviously that will vary on a game by game basis since some games may not need the functionality Steamworks can provide. However, not everyone wants to establish and maintain their own multiplayer service for example, and third-party options like Gamespy are a bit rarer these days.
Being on the Steam Store? Likely not as valuable as it once was for developers\publishers. Being on Steam(works) to make use of the features it provides? Certainly more valuable and a harder case to argue leaving, especially once embedded in. And some of the issues around game discoverability and the like on the store either are less pronounced or not present currently for the bigger entities left on Steam; such as SEGA’s financial success with porting Bayonetta, Vanquish, Yakuza, etc to PC.