I can see both sides of this!
I do hair extensions but I am not a trained hairdresser. I do agree that a 1 day course is definitely not enough to learn everything you need to know about extensions.
Before I did the course, I read page upon page for months, about hair extensions and human hair in general. I know how my hair is sourced and from where, down to cutting and blending etc. I also make my own hair extensions from bulk hair I have shipped over from a supplier in India.
It has taken me possibly a year to perfect my methods with extensions, for me, this wasn't about money making, it is something I am passionate about. But only hair extensions, I have no interest in any other aspect of hairdressing.
I do feel courses need to be longer and also stricter in who they let pass at the end. My course was the worse course ever, it was such a waste of money. But I don't agree that you need to be a fully qualified hair dresser. I know for certain I do better hair extensions than some fully trained hairdressers, it doesnt make much of a difference as long as you have the knowledge, skill and proper training.
I wish they would tighten up their training though, there is not a chance I would have let any of those girls who were on my training course come anywhere near my hair.
Oh, and I did my training with Belle Hair Extensions. Such a waste of money.