By what you are saying it seems that you really don't need a barbell either. Accually you don't need to buy any weights at all you could just 'do it like a caveman' and move stones around to build strength. Technically you still are a weightlifter :)
Yep. Ever heard of an Atlas Stone, Farmers Walk, Sled Pulls, Bale Tosses, etc, etc, etc?
Also I don't know of any weightlifters who dont do any bench press. There are ways to get around using a bench but it is inconvenient and removes some exercises including some important ones.
There are plenty of weight trainers that don't do benchpress. Plenty. And this just goes to show, as I said, no one piece of equipment is for everyone.
What if its time to change up your routine and you need 'normal' squats? What if all you want to do is press weight from the sticking point in your squats? What about people who don't want to have to work their grip to squat. It seems to be more trouble than its worth not to own one.
It's only trouble if you aren't creative. This guy isn't using a cage or a spotter.http://www.youtube.com/watch?v=XkK9-mnDAy4
And for a change from Front Squat (which doesn't require a lot of grip) you have Lunges, Split Squat, Step-Ups, and other unilateral work to be done. No to mention the others I detailed earlier. All without a cage. Your routine, growth, progress, and saftey are only limited to your understanding of the work to be done and your desire to get it done.
So technically there is no mandatory equiptment including owning plates, bars, etc but since no one wants to go to an extreme such as picking up boulders to get stronger a bench and a cage are high priority peices of equiptment.
Popular yes? Required? No. For everyone? No. Just like I said.