By this designation, I mean a stylistic unit where the writer thinks about a topic, mentions his thoughts, and perhaps pictures with them.
In my discipline I usually hear the term "essay" for this kind of writing. Such articles do indeed exist, but they are indeed qualitatively different from "research articles" or "literature surveys" (the other two major types of papers that appear in the scientific literature of my field).
Most of the essay-style papers I am aware of or was involved with were:
- Either: written by invitation. Some journals have ongoing topical "columns" for which a specific person is asked to either provide a short article every issue, or find somebody else to write a short article on the topic for an issue. These articles are almost always "essay" articles, reflections on a certain aspect of the literature. As you say, invitations for such essays are usually given to well-known experts, but these experts do often team up with (some of) their doctoral students or collaborators since the reflections of a team tend to be more interesting than the reflections of a single person. I was involved in multiple such papers during my PhD, but it's not really something you can trigger or plan as a student.
- Or: submitted to special tracks. Some conferences and journals have special tracks asking explicitly for essays and reflections (here is an example in the field of programming languages). If your field has something similar, submitting your essay there would be feasible.
So the question is whether it is worth it for a Ph.D. student to waste time writing down his thoughts on a certain topic (even if they are new)
That depends. I am a strong believer that you should not "tune" your PhD experience only towards what gives you to maximum number of output for your time - if you would enjoy writing such an article, believe you have valuable insights to share, and see an opportunity, then there is nothing wrong with going for it.
That said, I usually do not encourage my students to submit essay papers without an explicit invitation unless they really want to, since (a) the more custom a paper is to a specific venue, the higher your risk (if the one essay track in your community does not like the paper, then what do you do with that writing?), and (b) it is never quite clear how much your PhD committee or a future faculty search committee will value that kind of non-traditional writing (everything from "counts as a normal article" to "no value at all" is possible, depending on the person). That's fine if you spent a week writing up a short invited essay for a column, but maybe not so great if it takes you months to collect your thoughts, write and revise a detailed full-paper-length essay.
Of course, there are alternative ways to communicate your opinions - many researchers have blogs, and social media (particularly Twitter in my community) has established itself is the primary way to communicate opinions about the literature rather than primary research.