Today I came across a situation where I knew that I'd answered another similar question some time ago, but couldn't remember on which stack exchange site it was. I ended up doing multiple searches on several different stack exchange sites before giving up and just doing the research again - answering the question from scratch.
If this had already been implemented then it would have given me a single place to try out the different sets of keywords that I thought might help me find my answer.
Note, this question goes one step further than How can I list all my questions and answers across all Stack Exchange sites? and I can see that there might be implementation issues with it due to issues raised here, but couldn't the OpenID for the stack exchange account be used to filter the questions and answers from other stack exchange sites?