"Must" is a very strong word.
I despise everything that is the pen tool. Seriously. I hate it. Not because I don't know how to use it, like some people will try to convince me of, but because I just don't like it. Everything I've drawn in Photoshop, SAI, etc. has been done just fine without it, if not better, so to me, saying you "must" learn it is a bit of an overstatement (especially because in the video when they talk about whether or not it's necessary, short answer, no. It's a little contradictory).
Yes, the pen tool can work wonders, but it depends on what you're drawing, what style you're going for, etc. As someone who doesn't like to be constrained to fixed points and rulers, etc etc. the pen tool pisses me off lol
You can tell I have some strong beef to pick with it but I'll cut it here: it's not necessary. It's not a "must". Trying it out is a must, but you don't need to know how to use the pen tool or even enjoy using it to be able to create awesome work Saying it's a "must" couldn't be further from the truth.