I am an advocate of programmers starting their own blogs so that they can share their thoughts on programming and software development. But this is not necessarily a natural thing to do because writing is different from programming. The natural thing for a programmer to do is to write programs (duh!). So when you interview a programmer, a good judgment of whether they are good programmers is to ask them what kind of programming they do outside work. Do they have a hobby project they are working on? Are they contributing to some group project (open source or otherwise) online? If they also have a blog, that is a plus because it shows a passion for communicating with others, but it cannot be a filter.
On the other hand, when somebody is paid a salary for writing, i.e., they produce documentation or do some sort of technical writing, it just feels wrong when they do not have a blog or other outlet for writing. A person who likes writing will do it even if they are not being paid, because they have the need to put words to paper or the screen. So the equivalent interview question for such a person would be, what kind of writing do you do outside work? It doesn’t have to be technical stuff. Maybe they are writing a poem or a children’s story. But whatever it is, it demonstrates how much they love writing.
Similarly, a graphics designer without a personal website seems like an oxymoron. Does the person do the work only if they are paid to do it? Are they not interested in exploring design layouts, HTML, CSS and all the interesting stuff in the graphics world? Also importantly, how confident are they in showing the world what they have got in design skills?
So there you have it: The single most important question you can ask in an interview is about the natural thing for a professional to do outside their working hours? If they don’t have anything remotely connected to their profession, it is an indication that they don’t value the profession so much.
Obviously, there are exceptions. If a person is working in a sweatshop and only going home to sleep, or they don’t have the resources or are forbidden by law to tinker outside the workplace, that is fine. But in the normal course of events, you would always want to look at people who maintain their interest and use their time outside work.