I can see the argument. With the rear beam being stiff (relative to the ARB) and with stiff spring rates, the effectiveness of the ARB will be limited. My personal feeling (I was an engineer once upon a time, albeit not so much in the ride and handling area) is that I would rather use a softer spring with an ARB than a stiffer spring with no ARB. The primary function of the spring/damper is to control bump/rebound, not to prevent roll. By fitting an ARB (ideally an adjustable one), you can run a softer spring, giving you better ride control (to help maintain contact patch), while the ARB reduces body roll.
The Clio complicates it a bit further, as the beam itself obviously impacts roll stiffness, so you can also modify that with the above-mentioned stiffening kit. However, by doing that in conjunction with stiff springs, you are effectively "fixing" the roll stiffness and potentially compromising the ride stiffness. That set-up might work well in the dry, but could be too stiff in the wet and you could be left with something that is no fun to drive half the time. By running an adjustable ARB, you give yourself a means of fine tuning the roll control without having a dramatic impact on ride. Using the spring to control roll could mean you compromise ride quality. Of course, if the car is mostly used on track, where the surface tends to be pretty smooth, ride control becomes less important as you can get away with pretty stiff springs. If the car is used on the road, you have to make much bigger compromises and a stiff spring definitely isn't the one you want to make. Ultimately, if you look at pro-built competition cars, they usually run an ARB, as it gives further tuning options, so that's a pretty good sign of what the preferential approach is.
I don't have huge experience with Clio tuning, so someone who has tried out all the options could possibly advise better. Saying that, what works for them might not work for you. Some people like a car that feels loose, others don't. A FWD car with a skittish rear end can be really fast but, if you're not confident with it, there's no sense going for it, as you won't enjoy driving it. As with everything in ride/handling, there are a million different answers and any one of them could be justified as correct, so I'm sure some will have a different opinion to me 😂
