The point regarding the "unanswered" queue is a good one. It is indeed not ideal if questions appear as "answered" yet only have a partial answer.
However, considering the cost versus the benefit I think it is not likely that this will be implemented. Here, by cost I do not only mean the actual cost in developer-time but also the cost of added complexity both technically as well as sociologically (for lack of a better word).
The notion of accept rate, while once prominent, is not very predominant anymore. I would say a user that only got a partial answer can feel free not to accept it without being penalized by the system.
Two more points:
I would encourage users that only give partial answers to make clear this clear in the answer. (I does not have to be a flashy warning, but it should be explicit.)
For users that got a partial answer to a question yet not a full one, personally, I would consider it, at least under certain conditions, as admissible to reask a modified version of the original question stressing the still open point and taking the partial answer into account. (Of course the relevant post should have been on the site for quite some time before one should consider this.)