The use of placeholders should not be evaluated on a case-by-case basis, but instead on an overall basis. Understanding where placeholders can safely be avoided requires the same level of knowledge as knowing where 'strict' can be safely avoided. It is telling that nearly every Perl developer who has been lead on at least one project starts every file with use strict;.

The same goes for placeholders. It is a good habit. The query isn't using anything variable ... today. If you use placeholders, you have future-proofed your query. By not using placeholders, you have introduced a place where bugs can occur and you have to be more vigilant. I don't know about you, but if I can avoid having to be vigilant because a certain class of bugs simply cannot occur, I'm going to.

As for your example, if the snippet was part of a larger application, I would definitely say in a code review to use variables. There is no reason not to future-proof it.