Originally Posted by oneclick

I have.
The TA ones are notorious for getting stuck; they are chrome-plated pieces of cheese, as far as I can tell, the plating galls in the threads and the hex hole becomes a circle no matter what tool you use.
Exposure to maritime climes pretty much guarantees that this will happen, grease those threads when you grease the axle flats.
Opinion is divided on whether the covers are functional or decorative; that nice Mr. Jobst says they are good for keeping a loose crank bolt from falling out on the road - but a properly tightened crank bolt will not come loose - just like spokes or pedals or indeed any other threaded fastener.
(He does say that a rider who doesn't tighten them enough is also unlikely to notice when it falls out, hopefully we are all past that stage.)