I also ask this kind of question, and I agree with most of the other answers. Maybe it would help interviewees to understand WHY this type of question is important? Suppose we have an important business decision to make, and in order to do it, we need to build a new system. If someone runs up to you and asks what it would take to build a system that does X, can you give them an insightful answer that predicts the major challenges and resources required?
A junior programmer has no idea where to start. They are not ready to start talking without a detailed specification. A senior programmer will instantly see that there are many facets to the issue, and will attempt to hone in on a challenge. You don't have to architect every aspect, just identify an architectural challenge and then figure out how to address it.
Consider the issue of Google Docs:
One interesting thing is the shear scale of requests that will be coming. You can't just get a single server and deploy your code to it - this is a larger undertaking. A successful interviewee might zero in on this and will describe the types of resources that will be needed, and some of the technical challenges in implementing at that scale, with an application that not only has state, it shares state across multiple users.
Another interesting thing about Google Docs is that multiple people can edit at the same time. A successful interviewee will be able to discuss mechanisms for making sure the resulting doc isn't garbage, and a truly great candidate will realize that different methods of synchronizing or merging edits will have a big impact on performance and UX. Maybe even discuss variations: A shared document editor for writing code should probably use a different method of resolving conflicts than the typical Google Doc, because there are different consequences to things happening in a different order or having slightly different structure.
There's no single right way to create an app like Google Docs, you don't have to identify what you would do for every trade-off, but it's really great to find an area that has an interesting issue, and clearly explain what the trade-offs might be.