The problem with a nipple is it is difficult to prick the cahrge and difficult to clean the vent unless you remove the nipple.
George,
I didn't go looking for past threads, but I do know that there have been more than a few builds here where members have opted for musket cap ignition. I've been told that drilling a nipple to open it up would lessen efficiency, but if memory serves, some guys did drill nipples, so that pricks could be used without removing them.
Cannon Cocker,
The 209 primers certainly burn hotter than musket caps, and nipples are made for them.