The biggest advantage to the AOW cert is spending time with a quality instructor. That being said, it means you need to find a quality instructor to do the course. Many AOW class just breeze through the topics and do the dives. Others will give you more experience and confidence in your diving.
Exactly what he said.
To answer your question...
Yes it's necessary. If you want to progress to Cavern, Wreck, Dive Master, Rescue. It's necessary. If you want to dive certain spots, depths, it's necessary.
That being said... If you get a crap instructor, you waste your time and money. If you get a great instructor, it's well worth the money.