Hold downs (straps) have a prominent place because they are secure, when properly installed, and provide positive pull down to the bed. Disadvantages include that they can be difficult (or impossible) to keep entirely below the top surface of the stock.
Cams are useful for fast setup and easy positioning, especially for repeatable positioning sine they provide lateral force against a stop, and are also easier to set up so that they are entirely below the stock surface. They can, on the other hand, cause stock to tend to lift, and are more likely to work free than bolted straps.
Both have advantages and disadvantages, and neither is inherently better.