I'm personally more than happy to help my clients learn to trim. I think it's in the best interest of their horses that they know the hows & whys, and are able to give them more regular trims. BUT I do find that the vast majority of people who want to learn find that it's not quite as easy as it may look - both in terms of the hard physical work, horse's behaviour, etc & also getting everything right. Therefore, while they may end up employing me a bit less frequently, it doesn't generally do me out of a job!
Unfortunately while a good workshop/clinic & trimming 'recipe' may give you understanding of the basics, there are just too many variables & exceptions for a recipe. Therefore whether people do workshops, get lessons from the trimmer, whatever, I think it's prudent to keep booking the farrier regularly for some time & doing interim 'brush up' trims in between times, then when you have the hang of it, still employ the trimmer frequently to check/correct your work.
There are also a few different 'styles' based on different principles, not to mention better or worse trimmers, teachers, etc, so it's worth studying the theory, different ideas, people & pros & cons of them before deciding on who's workshop to attend.