I have a theory about the Painkiller.
Given the limitations of the Soggy Dollar's island location in the 1970s, I am certain this drink was developed with standard bar ingredients of the time, including canned pineapple and orange juice.
Fresh juice is too sweet and lacks thrust. You need the acidic bite of the canned juice in order to cut through the sweet coconut. The acid is the tip of the marlin spike, to use a naval metaphor.
Try it both ways, I think you'll agree.
@Chris Dimal
Okay so this is interesting. I haven't had Old Monk before, but I imagine its flavor profile is not nearly as intense as Navy Rum. That could explain why you enjoyed the fresh juice over story bought.
With Navy Rum, the added zing from store brought will help to pop through.
But with a less intense rum, the more delicate flavor profile of fresh juices could play nicely.
Cocktails are fun.