IMO, if your using something like BlackHorn 209 or one of the other powders made for inlines, the 209 adapter might be worth the trouble. Those powders need the extra heat from the cap to be really reliable.
For those shooting Pyrodex, 777 or real black powder IMO the 209 adapter isn't worth the cost and bother.
Did I say bother? I've heard from more than a few hunters who dropped the cap for a 209 adapter when they were trying to reload out in the woods. Your lucky if it falls into the snow because you can see the hole it made. That makes finding it possible. Dropped into a pile of loose, dead leaves can leave you spending your hunting time looking for it rather than hunting.
I think using a regular #11 cap is probably a better idea. If you drop it in the leaves, no great loss. Just get another one. As for using a regular #11 cap with something like Black Horn 209 if you take a few minutes during your first load of the day, you can unscrew the nipple and put a little powder into the threaded nipple hole. Then reinstall the nipple, place a cap on it and your ready to go.
Those few granules of powder under the nipple will easily fire from a #11 cap and the flame they produce will fire the main powder charge. By the way, doing this "under the nipple" prime is a good idea even if your using real black powder to hunt with. When that first shot MUST fire, it's worth the little extra effort to do it.