I'm not too crazy about those type of summaries, but I don't mind them if they at least point to what is going to take place in the fic. Like in your example, I would expect maybe, after a hard day of spellcasting, Merlin is going to get a nice full body massage and some hot handjob action from Arthur and then fall blissfully asleep. But if the story turns out to be them going to fight a dragon, or something, then I would really wonder WTF.