soft-question tag is useful for people who want to see all soft questions, or who want to filter out all soft questions. Although I don't know about the first category, 97832123's comment confirms that people in the second group exist.
The main reasons I can think of for not having such a tag are:
It is too vague. It incorporates many different topics: pedagogy, book recommendations, math software, and gives them all the same tag.
I think that this is not a problem because any such categories that grow large enough will get their own tag. See the 14 questions tagged books.
It adds little information about the question. To which I would say "it adds some information." If you see a question tagged
algebraic-geometry on the front page with a fairly unhelpful title, whether or not it also contains a
soft-question tag may guide whether you click it or not. In this case, I think the tag is doing its job.
It uses up limited space. Questions only get 5 tags total. If I spend 3 on
books, that only leaves two tags for real content! If I want to ask for a list of books for teaching calculus in a physics context, then what do I do?
This I think is a real problem, and a possible reason to consider restructuring the tags. If we can think of some synonym set that would achieve this, though, I'd rather support that than removing