That seems to be banking on the assumption that every place you raided would have someone available for capture who'd know how to mend and maintain boats. I doubt every English monastery was full of ships carpenters who could replace your own dead craftsman for the voyage home...
Even if you were raiding a coastal village that did have a local with the necessary skills, what if that local was killed or fled before you managed to capture him? You'd be out of luck unless you still had one of your own alive who could do that job. Better hope nothing bad happens to your vessel on the trip home...
I'm not sure what the concern is. If the ship is in such poor condition that only a skilled boatwright can keep it afloat for a few days until we all get home, you shouldn't be using that boat anyway. If it's sound enough to get you to your target, you can get home in it without a trained carpenter. Any leak you spring at sea, or broken spar or whatever, is going to be hastily patched with materials on hand in any case, and anyone who has been on a boat more than a once or twice will be familiar with those procedures. If *no one* knows how to get a ship home, that's called "suicide". It's literally a ship of fools who will probably all die at sea anyway.
Yes, skilled men risked their lives in battle, that was just part of life. And "professional soldiers" really didn't have a lot of military training beyond the standard militia muster, anyway, as far as we can tell. Sure, they probably spent a little more time at weapon practice, but it wasn't like comparing a modern couch potato to a Ranger or Navy SEAL. They were mostly just guys who were more inclined to fight than other folks, and were getting paid to be ready to do it.
But again, if your whole raid depends on men who are too skilled to risk in "combat" against those unarmed monks, you've probably already lost. Not every raid succeeds! Sometimes raiders get killed and their buddies run away. If it weren't dangerous, everyone would be doing it, eh?
Matthew