This topic seems to come up quite often in the makeup community. I have seen some pretty strong opinions on the subject and wanted to share my two cents.
I attended Westmore Academy for Cosmetic Arts in Burbank, CA. It was a wonderful experience and I learned a lot. I can confidently say that it was a good investment for me and it made me a better makeup artist. Now with that being said, I don't think one needs to attend makeup school to become a makeup artist.
Don't get me wrong, school could be that extra bit of training needed to fine tune some makeup skills or learn the basics (which I needed). The reason I decided to attend was that I didn't have a variety of makeup experience. I am the type of girl that wears very little makeup. I would say 90% of the time I go out with absolutely no makeup on at all. So for me, I wanted to learn the basics (especially foundation, skin matching, etc...). I also found the business lectures/discussions very helpful. The instructors were makeup artists actually working in the field (movies, photoshoots, runway, music videos, etc...) so they gave us lots of tips on how to market yourself, contracts, agencies, and lots more.
Now for someone who has experience (applies makeup regulary on themselves or anyone else), I think they could easily become a makeup artist without going to school. There are so many self taught makeup artists on youtube (and in general) and they are amazing. I look up to many of them because of their makeup skills.
Makeup artisty isn't something that can be taught in a classroom and the individual instantly becomes a makeup artist. It takes work and practice. Some people have natural talent and that will get them far. For others, school might be needed to give them some new techniques and inspiration. In any case, I think it is really up to the individual to decide if its right for them. I don't think it is necessary. But its a nice extra to have if you have the extra money. I won't lie; school was pretty expensive so it was an investment for me.
Let me know your thoughts on this subject in the comments below. And feel free to ask questions if you have any. =)