That is a fairly common point of damage, though I wouldn't say it plagues a large percentage of used kegs.
As long as the dent isn't changing the geometry of the O-ring (like pinching it) the post should work fine. You might be able to remove the o-ring and use pliers to straighten out the dent, but it looks like there's a fracture line that might just give up, so I'd be inclined to leave it as is as long as it's not leaking...
Cheers!