[SamReidHughes]

Treating the bags gently takes more time and effort. For starters you don't know heavy the bag is, so first you have to figure that out, and you have to spend effort managing its downward trajectory. It's more wear on the joints and body to handle it carefully than to just throw each bag with good form.

I don't know what I'm talking about, so treat that with a grain of salt, but it's plausible.

[Moru]

It's the other way around, less wear if you are careful. Tossing around puts more strain on your joints.