WLFs are common on square format cameras, meaning 6x6 usually, and on cameras with rotating backs (RB67.) Rectangular 6x4.5 and 6x7 cameras without rotating backs are more commonly used with prisms.

I use the built in WLF on my Yashicamat 124 TLR and the AE prism on my Mamiya 645.

I agree that the laterally reversed image can aid composition, just as the upside down image on a view camera ground glass can, though it hampers following a moving subject. One adjusts, but still sometimes takes a second of moving the wrong way, especially if, like me, you also shoot other cameras with prisms.