Free float means the handguard can move from hand or sling pressure - and the sights don't. That only works when the sights aren't mounted on it, but stay on the barrel.
Mount the sight on the top rail of a float and it's going to move around again. That's one reason why railed handguards are dropping out of the market. Not being able to bridge the upper to handguard joint for a scope is another. That should highlight the issue even more - the scope makers won't warranty scopes that were partly mounted on the handguard - because they will twist and bend the scope.
If it moves enough to screw up a scope, it will move the sights, and the sight picture. If you are using a red dot or scope mounted on the upper, then the need for some form of BUIS is extremely rare. Close combat shooters and few comp gunners do it, they get them free, for a reason. However, most soldiers in combat zones aren't bothering, and the three gunners are sponsored i.e., they are helping to market stuff.
It's all about the marketing. And it's not necessarily something we need to spend money on, which means I'm going to have more spare parts for another build when I take off my clamp on front sight and put on a free float. With a new stock I think it will trim a pound off the dead weight.
