I don't think this is practically much different then answering one thing. If you give one answer and 3 "I'll get back to you on that"'s-- this creates a promise of a future asynchronous answer, which is only as good as your word. People often have too many tasks, so to get those remaining items on your queue, they'll have to ask you again.
As the recipient, it's more challenging to receive the future promise of an answer with no SLA.
I would be to differ. To me, there is a large difference between just ignoring 6 out of 7 questions I asked you or you telling me that you do not know the answer right now but will get back to me.
I agree that if there's no explicitly stated SLA and no implicit SLA given the relationship history between the two of us (e.g. I might know you're usually going to get back to me within 24 hours on such items), then this is practically the same.
I do not operate under such circumstances though. If I tell you that I will get back to you, then I will get back to you within a reasonable time frame and you will know from our previous interactions that I'm good for it in most cases and that it's totally OK for you to ask again after a day because I might have forgotten. I'm not perfect.
Since this was an example answer only, it is also possible that for one of your 7 questions the answer will simply be that I cannot get that answer to you within any reasonable amount of time at this point because of other priorities I have and that you should find someone else or I might point you towards someone else. In any case, you will have all of your 7 points answered. I won't just ignore them.
The difference is as a sender I would know that you parsed each of my questions, understood them and decided to either not answer them now or just never answer them. Replying to one is ambiguous, if it was actually important it just leads to having to follow up again, restating everything that wasn't acknowledged.
As the recipient, it's more challenging to receive the future promise of an answer with no SLA.