Possum,
It's not a must, but they're cheap (relatively - $5-6 for a 1L bottle), extremely durable (if you get the lexan sort), and very versatile. I use mine while hiking for water, oatmeal, soup, granola, and the occasional cocoa. They're great.
I just sent my review in. Hopefully will be getting the bottle soon. Can always use more of these. 🙂
Rob