I wouldn't be so quick to dismiss straps as placebos. Colbybruce's pics of old school strapless holsters show they don't have slide guards at the back, and thumb snaps automatically provide that in the form of the tab there. More coverage of the outer side of the pistol is also provided. There's nothing that says the snap needs to be done up, either.
The OP's point is well considered and surely something carriers are wondering about, even if unconsciously, when buying. And some employers, such as police agencies, absolutely require them. And for 1911 carriers the cocked hammer can create some unease even for observers.
Many of my designs are strapless with tension screws -- that DeSantis in Colbybruce's post is one I did for them -- because the cognoscenti are my target market. By definition, then, they are not for everyone.
Crazyphil: Chic didn't use tension devices.