Open or concealed, doesn't matter - there are reports of concealed carriers having their guns taken as well - as I said, this is NOT an open carry issue, rather a carry issue.
Here's just one recent and local example:
Conceal and carry permit holder robbed with own gun | kare11.com
It could be argued that had he been open carrying the attacker may have chosen a different target, or worst case - the outcome would have been the same.
A study was done and found that you are 60% less likely to be attacked if the person knows you are carrying... Considering the chances are already pretty low, adding another 60% less of a chance to that is OK in my book.
We will never know how many instances open carry prevents, but there have been cases where it has prevented a crime.
We do know that concealed carry prevents 0.
We will also never know how many times an open carrier becomes 'the target' simply because they are open carrying.