Watching a guide should be done if they say in the description "watch a guide" or "know this mechanic" and watching a guide is really common, but other parties advertise as doing it blind.

One reason to do an extreme blind is if you don't think you will need a guide based on the mechanics in the normal version. I did this with Innocence because I was fairly confident there was not a lot they could do to that trial that I wouldn't be able to figure out on my own and I was right. I thought that the tankbuster is obviously going to be a tank swap and it was more or less what I expected.

When you have played the game for a really long time, you can know how to get a lot of mechanics fast without a guide and will often be able to wing them on the first try, but there can be exceptions. In the current savage tier for example, if you try to do devour without any sort of guide or clip analysis, it results in headbanging and dying over and over, because you need a mental plan of how the aoes work and how to move. Another example is Tsukuyomi, where if you haven't come up with a genius plan of how to handle the meteor drops like guide-makers have, you will just wipe the party over and over.

You specifically talked about when someone queues for extremes. At least in the NA region it is not common to queue for extremes except some of the old ones that you get in Mentor Roulette and the people who queue for these are sprouts who don't understand how to prepare properly for an extreme trial or that they even need to.