It sounds cool, but remember, you could learn True Polymorph and make a clay golem every day. Or learn True Resurrection and bring back a legendary hero (or big bad, if you prefer). Or Gate and summon your opponent into a room full of Glyphs of Warding just because you know their name. Or Wish and make a new Clone or Simulacrum or bind a demon for 100 days or whatever other crazy exploits you can find. Is being able to deal a bunch of damage from a mile away once a day really worth it?
Imagine living in an RPG verse where all the players have superpowers, but all the NPCs can get them to do whatever they want by calling it a "quest". And also can pay with premium currency. Seriously, how is the kid asking me to feed ducks paying in primogems?
And it's good to keep a little, just in case you have the opportunity for Clone and don't want to cut out more.
I wouldn't expect you'd just keep a rotting head though. Does it still count as a creature for the purposes of Gentle Repose? Do they just not have a good way to get rid of the flesh and they want to keep the skull?
If they keep taking the slow sheep, they'll evolve to be faster and make the whole thing less convenient. Dragons live a long time, so they know from experience that in the long run it will be easier to only eat the fastest sheep.
Ophanim only have two wheels and they don't have wings. Why are biblically accurate angels never biblically accurate?