If a question can not be answered because it's about the preference of the readers, shouldn't it be closed? Some recent questions include "Should I study Latin in English or in Spanish?", or really anything you find with the search of "best translation." Most questions like these are closed, but some are not cared about. Are there certain questions that are okay, but others not at all? Sometimes these apparent rule-breaking ones are accepted and answered seriously, and they aren't resource questions. They're real inquiries that I think simply don't belong, and should be discouraged. Is this okay? Are they actually considered resource questions?
First of all, experience is not opinion. The question you link to would get a wonderful answer if someone has experience with both Spanish and English material and writes up a comparison. Personal experience is an important form of information, and SE is a great place to share it.
Many of our questions and answers include opinions as is. And that's fine. If you ask for a Latin translation of "I forgot to take the dogs out this morning", you will probably get different suggestions. People have different opinions on what is the most natural way to render such a phrase in Latin. Asking for a translation of anything (a single word, an idiom, a phrase…) is asking for an opinion.
In my opinion asking for opinions is allowed, and I would even encourage it, but only if you ask for professional opinions. Professional opinions are important content for our site, personal opinions are not. Professional opinions are very important in any field. I am a mathematician, and mathematics is often considered a purely fact-based and objective field. But that is not really the case; experts have opinions on what is interesting and what kind of tool is likely to work in different situations and how things are (more or less formally) related, and such opinions are extremely valuable.
Expert opinions (stripped from personal opinions if possible) are interesting and useful to a broad audience of people interested in Latin. I certainly benefit greatly from hearing the thoughts of an expert Latinist on a matter, and the benefit will be even greater for someone with no or very little education in Latin.
Professional opinions can often be explained, and that adds value. Pure opinions are less useful. An opinion question should not be an opinion poll, but a request for opinions on a well defined question and the reasoning behind those opinions. (Voting works like an opinion poll to some extent. That's useful too; people can show their support and agreement of opinions and the way the opinions are presented and justified.)
A good opinion-based question is answerable, and a good answer to such a question explains what the opinion is based on. See also this SO meta post from 2010 about subjective questions.
On the other hand, it is possible to ask too opinion-based questions. They tend to lack focus, be too personal, or ask for a more personal opinion. Such questions have been rare on this site so far.
If all opinions in all forms were banned, this site would not be here at all.