I would only go adjustable gas on a rifle length barrel for these reasons:
1. Gas port is too big and gun is overgassed (brass going forward of ejection port door, usually)
2. Using a suppressor (will cause gun to be over gassed with can on)
3. Rifle is a race gun/3gun type rifle with a low mass BCG and low mass buffer system.
In the last instance, people are also often reloading for the gun. So tuning gas is nice).
If your barrel is properly gassed, and is ejecting the brass you use smoothly, and you have a decent brake, you are chasing small diminishing returns after that. As Combat Diver noted, rifle length gas is very smooth feeling to begin with. And, with some heavy barrel rifles, a brake ain't doing much as the muzzle stays pretty flat anyway.
A good brake is going to do 90% (or more) of what people want for recoil reduction.
The Low mass system with adj. gas block will reduce the impulse of the BCG and Buffer unlocking and going to the rear and bottoming out and then the impulse of it all returning and locking back up, which can be exacerbated by heavier buffers/BCGs.
I like having adjustable gas on my match rifle. Just want every ounce of control and flatness I can get. And, it's a very light rifle so it makes a very noticeable difference. I do have other rifles where it wasn't really needed, and they have regular GB's. On the barrels I have used adjustable blocks, I always set it for my softest ammo, and leave it alone. Once it is set I don't screw with it too much. Just my non-expert opinions.