I think they should be off-topic. All of the same problems mentioned in that blog post exist with Music shopping questions. The problems include:
- "Bad subjective"
- Answers amount to unverifiable opinions
- Two valid answers could be complete opposites
- Too localized
- May become quickly outdated (not true for theory books, perhaps, but certainly for composition software, certain equipment, etc.)
- Often relevant only to the asker
- We can't possibly answer the question well without knowing the user's learning style, background, preferences, etc. which is not easy to convey in a short text question
- The user should learn to evaluate things on their own ("teach a man to fish")
Regarding the last point, questions that ask how to evaluate things are better. Example from the blog:
Q: How do I tell which point-and-shoot cameras take good low light photos?
A: I strongly recommend looking for something with
a fast lens (2.0 at least)
reasonable ISO handling (at least 400, but preferably 800)
the biggest sensor available
The sum of these factors are really critical for low light situations.
I'm sure you can see how a similar question about evaluating guitars for a particular application, for example, would also be a good question.
Of course, not all shopping recommendations will suffer from all of the above problems. But they will all certainly suffer from some of them. There's also the question of what we want this site to be. Do we want it to be a place where we talk about music practice, performance, history, theory, and composition? Or do we want it to be a place where we tell people what to buy? They're not mutually exclusive, of course, but it will be much harder to attract and keep users knowledgable about the former when they're deluged with shopping advice.