You have asked a lot of questions in your question:
So, why not listen to their opinions?
We would be silly not to. We have no issue with users giving their opinions as part of their answers to focussed objective questions. It is when opinions are asked for that I vote to place questions on hold, because that makes the question open-ended.
Wouldn't it be enlightening?
It certainly can be, and if opinions are given that appear unfounded, we have a post notice that moderators can use to ask answerers to provide references to back up what they are saying.
Where should you post a GIS opinion based question if not here where you could probably get the best opinions?
Questions seeking opinions are welcome in the GIS Chat Room which is the place within the GIS Stack Exchange set aside for this purpose. If it is software recommendations, hardware recommendations or open data recommendations that you want then we also have specialized Stack Exchange sites for those.
Why do we have to avoid questions leading to opinion-based answers?
What's the best desktop GIS?
I think it's X, user A thinks its Y, user B thinks its Z.
We are all entitled to our opinions so there are at least three correct answers to that question, and maybe many more.
The idea of focussed Q&A is to ask a question to which only a single answer is anticipated. It is desired to have multiple answers posted on each question but this is only so the clearest and most comprehensive of the alternative ways to solve that well scoped question can be offered and voted on.
We do not want answerers to be encouraged to base their answers on opinions. We want their answers to be supported by evidence. Closing prevents answers being added. Once the question is re-scoped to a single focussed question that does not solicit opinions it can be re-opened.
Why not simply let the voting system do the job?
We do not want GIS Stack Exchange to be turned into a harbour for polls.
What's your favourite colour to put on a map?
Who is the most influential GIS blogger?
What GIS does raster best?
I think X GIS sucks, do you agree, and why? Such questions go rapidly out of date, can be divisive, and the community has shown little desire to maintain them.
if you navigate through this site, you'll find many great questions
closed or locked as primarily opinion-based. Some of them even in the
top voted questions of all time for this site.
Some of these questions appeared great to me when they were first asked, and while I had no experience of seeing what happened to them over the years, so I can understand why users new to the Stack Exchange network might initially think the same.
However, I think there is no comparison in value between questions that everyone has an opinion on (and will therefore vote on and give a multitude of answers to), and a focussed objective question with one or more clear objective answers that has affected the work of someone today and is likely to do the same to many others in the future. It is the clear objective answers to clear questions that I value on this and all similar Stack Exchange sites.