"Need"? definitely not. A lot of people never clean their ARs at all and never have a problem. They may be dirty as hell on the inside, but people just keep on shooting them.
Most people likely crack it in half, break down the bolt carrier group, clean those parts, quickly clean the rest with oil and Q tips, and run a pull through down the barrel a couple times.
I would say that very few people ever completely take everything apart to clean it (gas tube coming off, every single lower receiver pin and spring out to fine clean, etc). There just isn't a need for it.
Most modern ammo is fairly clean and it's very rare that you'll get enough buildup to actually cause stoppages, so most people just clean it because they like to take care of their stuff.