They could've wanted "strength, magic, and courage" and just didn't realize that the when the spell said "straight magic and courage" it actually meant Arthur Merlin and Gwaine. Or if you want to keep it so spn isn't set in a specific time in the show, Morgana might have summoned Sam and Dean and try to get them to kill Arthur (crappy idea, I know). Maybe they need Excalibur to kill some sort of monster (which wouldn't really make sense considering they already have the sword that dean pulled from the rock), okay scratch that. They could need the cup of life, so Cas might transport them back to Camelot to get it. I feel like Charlie would be super pissed that she didn't get to meet Arthur and Merlin also, so the entire time I'm reading Men of Legend that was the only thing I could think of.