I live in California where there is a bullet button where the magazine release button is supposed to be. I have heard from three gun competitors in other states that the reason they stay away from them is because it can interfere with the release.
Anyways the core concept is speed of operating the controls and the three gun guys do just fine without it because the practice their order of operations to the point they don't think about any step.
In a normal consumer setting it is just convenient. The only time I'm truly grateful is when I'm varmint hunting with the bolt open so as to be safer than sorry.
It's a solution looking for a problem. I've spent 4 years overseas in combat zones and can honestly say I've never seen one, never heard anyone say they needed one and would probably get made fun of for having one.