the bag itself provides good protection, I wear overalls all the time, sleddogs constantly jump on me, its surprising how often they hit the stoma directly.    i even had a bag tear from a sharp claw once.

I would say that a two piece appliance can offer some protection to stomas that don't protrude very much, but it's risky to rely on that if you're stoma is being subjected to impact, pressure, or sheering. 

I've taken a few unexpected hits to my stoma and the nausea and bleeding that followed was not nice at all. Guards can do a great job when someone's stoma is at the belt line, too. 

I've always read that when you sneeze to hold in your stoma to avoid hernia, so i push mine flat when I sneeze.

I'll testify to holding the stoma with a cough or sneeze!  I had a really nasty cold at the 1st of the year, I honestly thought I'd torn something up inside from all the coughing I did.