You are forgiven for thinking that opinions have no place in academic writing. Some folks think that academic discourse has to be objective and non-personal, and the phrase “that’s just an opinion” is used to dismiss ideas. I don’t like the disrespect shown to opinions when the word is used that way. Human beings are highly opinionated creatures, and intelligent humans are full of informed opinions. That doesn’t mean the opinions are guaranteed to be correct, but they are also not guaranteed to be wrong or unsubstantiated.

A person’s informed and educated opinion is the start of the process that results in a strong argument, so they warrant inclusion in the model. Students tend to take classes in subjects that they are interested in, and they bring their own unique set of skills, knowledge, interests, and personal views to the table. When they receive a topic or research question they are likely to form an opinion quite quickly, and the final argument will likely be an expanded version of the opinion.
The student takes that opinion to the library (or Google) and begins to flesh it out and test it. Ideally, the opinion will grow and morph into a good argument as facts and new ways of looking at the world are discovered. When worldviews and personal beliefs are applied to a topic they become the premises of the personal opinion. People’s premises tend to be personal and relatively stable, so those will rarely change during this process. Yet they should be challenged and developed in a way that sharpens their use. A humble person should be willing to accept that their opinion is wrong, but retaining your original opinion throughout the creative process is not necessarily a sign of arrogance.
To reiterate what I wrote in the previous post, opinions are personal, and arguments tend to be based on the writer’s opinion. So when marking a paper and critiquing the argument, be aware that you are likely touching something in which the student is personally invested. Be clear with the critique. Are the premises and argument mismatched? It is under-supported by the research? Is the argument sound but the paper does a poor job of explaining it? Is the argument insufficiently aligned with the historical record? Work with the student to help them get better at developing strong arguments and be clear that you are on their side.