"I'm not sure if you are advocating it, exactly, but I don't think writing big-budget fantasy specs is the best way to attract the attention of an agent or manager. I think it is widely regarded as one of the hallmarks of a beginner, as they are a notoriously difficult sell."
Well, you think. Didn't someone else just say that opinions are like a-holes, everyone has one?
I didn't advocate anything at all. I did say what you've written will determine who wants to read and represent your material. And that as far as I can see, it is easier to get a rep to read a horror or a fantasy than it is a gay romantic comedy (independant). From the agent's view point, both horror and comedy are more SELLABLE than a Latin gay comedy.
Make of that what you will.
I would say that in my case, of the producers that I've come across, it does appear that ultimately decide or reject on the story. If it is decided that changes need to be made to the character's or the story, they'll have you or other screenwriters do that.
Actually, with people that they know, a lot of people are not willing to give an opinion. And for good reason. Strongly criticize someone's work, and you two may never speak to each other again. Even when I was a script reader, we were told not write overly harsh evaluations about any scripts. Because evaluations have gotten back to writers, and enemies are made. If you must get your work evaluated, you may want to go an experienced script consultant, or take classes. People who have a business interest in you won't nsult, will be more diplomatic in pointing out areas you need to improve, and if things don't work out, well, this isn't a friend or a relative that you'll be pissed at.
I do think people here have a tendency to make everything sound harder than it is. When I worked as a script reader, the development execs told us that they wanted us to make the decision on the story. As far they were concerned, the scripts were all first drafts that had to be rewritten. And development execs do tell readers to pass along scripts that fit the perimeters of what they are looking for, budgetwise and genre wise.
If it falls out of what the producer or agent is looking for, the reader will always give the story a PASS.