I had access to a copy about 2 years back and was pretty overwhelmed by it. Then, last year, I started building bows and had read a lot more stuff and followed lots of good threads on the boards. i broke down and bought a copy, feeling that if nothing else, as a builder i should have it in my library. I got quite a bit out of it this time through, and figure I'll get more next year, with more experience behind me. It's certainly not a "sit down and read and digest, then go build a bow" book. However, there's a ton of information to pour over, and over, and over, getting a piece of the puzzle each time. I've found myself going back to it for specific information look ups a number of times.
I'd say to ask yourself where you are and how serious you are. If all you want is to build yourself a few simple longbows, I'd label it "nice to have... could learn some basics from". If you want to get seriously into the science of reflex/deflex and the physics of bows, then I'd label it a "want for sure".
I'm strictly into simple longbows, so for me, was it necessary? No. Much of it is beyond where I will ever be, but I've gotten some things out of it and I'm glad I have it. I dip in every now and then, and who knows, I may grow more than I expect to. Of course, I'm a book nut and always like to have a good library on anything I'm doing.