I for the most part agree with you: The vast majority of "advice" questions that I see on this site are just vague (and, let's be honest, poorly-written) calls for open-ended speculation and opinion rather than practical and/or answerable questions that are useful to anybody (anybody including, in many cases, the questioner himself). I close many of the blatantly bad ones when I see them, but for the most part it's up to the community to deal with these appropriately (or not, as the case may be).
Advice questions aren't inherently off-topic on Stack Exchange, but they're tricky to get right because they are highly subjective where the Stack Exchange model has always been geared towards objective questions. For the canonical blog posts on what makes a "good" subjective question and what makes a "bad" subjective question, check the following links:
We as a site really don't have any guidelines on how to deal with these sorts of questions yet. There have been a few proposals been made over the years, for example…
…but the problem is that the community as a whole still hasn't really stepped up to enforce any of these. Even now, three plus years into Islam.SE's beta, I still find people encouraging sloppy advice questions by upvoting and answering them.
One thing I can tell you, however, is that a lack of fixed guidelines does not give such questions carte blanche here: The general close reason of "Primarily opinion-based" is applicable across the Stack Exchange network for good reason, that being that the Stack Exchange model is plain not designed to handle these sorts of questions.
It is important that poor questions be handled quickly, and the onus of that is always on the community. If the community as a whole wants them to be on-topic for whatever reason, it is still of vital importance that they be constructive; this often means that poor questions either need be edited to work under the Stack Exchange model — or put on-hold entirely — before they attract poor answers.
On-topic or no, just because a poor question hasn't been closed (yet) doesn't mean it's fair game: Posting good answers to poor questions is just likely to encourage more poor questions in the future, whereas posting poor answers to poor questions is more likely to attract flags and/or moderators who can leave naught but devastation in their wake. Neither of which is really a desirable outcome for this site.
So, in short, no, there are not yet any clear guidelines (at least none that the community seem willing to accept) about how to deal with these beyond the general Stack Exchange guidelines for subjective questions (see: https://islam.stackexchange.com/help/dont-ask):
Some subjective questions are allowed, but “subjective” does not mean
“anything goes”. All subjective questions are expected to be
constructive. What does that mean? Constructive subjective questions:
- inspire answers that explain “why” and “how”
- tend to have long, not short, answers
- have a constructive, fair, and impartial tone
- invite sharing experiences over opinions
- insist that opinion be backed up with facts and references
- are more than just mindless social fun