I was reluctant to use a shell seat at first. I thought it would be to hard and be very uncomfortable with any extended seat time.
When I bought my existing car it came with Sparco seats. Once experiencing the shell seat I don't even like the idea of getting in a car without one. For me I feel much more secure, I am one with the car, much better back support, and once I set the lap belt it is right for the whole race.
I had back surgery about 20 years ago so I am very sensitive to my seat adjustment and how my belts fit. The shell seat eliminate the problems that I had with suspension seats and it is no longer an issue.
There have been other threads about this, I like both for diff reasons. I have suspension seats in my sandcar as it does not have high $$ shocks and makes the ride better, but I am always adjusting the belts, (and they have the clips). In my 6100 truggy which has very good suspension, I run the hard seats (Corbeau fx1 for me) and belts stay tight and are comfortable and containment seems better to me.
I like everybody used suspension seats for the first 10 years of my racing and I always was adjusting the belts my chest would start to hurt after few miles. My 12 car had sparco's and I was really apprehensive about using them but my first race with them I could not believe how comfortable they were. My 10 car has Cobra's in it and I love them as well. I spent 9.5 hrs in the car at the 500 and I didnt touch my belts once. The problem with suspension seats is that when the seat compresses your belts will loosen and when the seat returns it pushes back into the belts possibly in an awkward position.
The shell seat keeps you in position and strapped in tight. If you are getting bounced around in a limited suspension vehicle it would be just as important to have hour body held in position. You have better vehicle control and less distraction while driving.
Its one of those things that once you have one you don't know how you ever did without.
How much control is necessary? I had hard shell buckets in my swingarm Baja Bug 40 years ago, sure was nicer when I could afford SuperSeats...like adding six inches of travel all around that I couldn't afford.