Who publishes commentary papers, and how? In my area of research (experimental psychology), publishing an empirical paper takes on average one year of work. It seems to me (I could be wrong) that those authors who appear to use every available opportunity to write commentaries on recent "big" papers have it easier (or so they think) than those who instead prioritise publishing their own research despite the increased required efforts. 

My first question is to do with the "tactic" of relying a great deal on such publications in order to build one's CV and/or h-index. While a commentary paper does not, of course, "weigh" as much in someone's CV as an empirical paper, it might still weigh more "per unit" [of time spent working at it]. That is, one could conceivably write such a paper in a month, whereas the "academic credit" you get for it, while lower, is certainly not lower by a factor of 12; and in any case the h-index is blind to the distinction.

My second question: in what circumstances is such an offer (made to a relevant journal, to write a commentary to a "big" paper) likely to get accepted, and by whom is it typically made? 
If an early-career researcher feels they have commentaries that the community/field might benefit from, should they pitch around their commentary-paper idea to journals? 
I used to think such papers are something only established researchers write, and at the invitation of journal editors rather than at their own request. However, I am seeing more and more junior researchers who are writing (unsolicited) commentary papers with only a minimally-relevant publication record.
I'm not from your area of research so my answer might be invalid, but in my field it happens that you are asked to list "original papers" and "derived papers" in different sections of your CV. So when you are applying, e.g. for a professorship, the original papers are the ones which are taken into account more heavily than the others. 

Especially earlier in your career you should focus on your own scientific work. This qualifies you to write good commentaries, later ;-).