I entirely feel you within this subject, and I see you're reasoning, which is basically just 'If you're going to accept the request, why not actually do it?' or, 'If you accept the request, and are unable to do it, why can't this be explained?'
I understand how frustrating it can be when people, especially when active, do not go through with your requests, or even worse- commissions (and do not other refunds). On one hand, knowing a person is active and entirely capable of using their time to fill out your requests can be frustrating even after they've confirmed they can't do it (but hell! At least they told you that!), but it is an entirely different story when 1) they are active. 2) you have contacted them in regarding your requests and whether or not they can carry it out, as well as how long it might take, and 3) receiving no reply whats-so-ever to mere questions; not demands that they get your little project done.
While as Krimzen mentioned that one of the problems itself is that it is a request, thus done for free and the person taking them up is receiving nothing other than the joy of bringing out the results and presenting a finished project to you, let's put it this way:
If you're going to accept requests, then maybe you should do them or say that they can't be done, as you are either 1) currently unwilling to do so (which is, once more, entirely understandable to say; because as these are requests, you are not receiving any currency in exchange for the project. You're just doing it out of the goodness of your heart.), or 2) Unable to do so, or not having the energy to bring out the results you'd like, or the results you think would be most appreciated.
But as typed unfortunately before I could post this (I was ninja'd yay), whether a request or not, you shouldn't give them something you made in five minutes and said 'here you go i made this be grateful'. All-in-all, I suppose it depends on who you are requesting something from on how the results will turn out.
As rude as it may sound, I for one would not spend as much time on a request as on a commission. But it's simply nice to give an at least half-hearted effort into the completion of the project, and if it can not be completed, merely the existence of an explanation.