I think there is a spectrum between playing to win and playing to learn. For the former, you should play the style that you are most comfortable with / best at - for most players (myself included), this leans to the more defensive side, and with good reason - most people aren't very strong at attack, and that is why many shufflers get to platinum. I posit that any player above say, mid-silver could spend a few weeks practising a purely shuffling technique, and use that to get to platinum. If your goal though is to maximize your learning, it only makes sense to play the styles that you're least comfortable with - for me, probably hyper-aggression.
Contextually, I would think the natural circumstance-goal pairings would be:
Tournaments: Play to win
Unranked: Play to learn
Hence, I think if there's ever an appropriate time to shuffle, it's in a tournament game.
P.S. I too find it frustrating and unsportsmanlike for people to only shuffle, but that's only because I'm one of the majority that are weak at attack. If I were a strong aggressive player, I'm sure I'd warmly welcome a game with a shuffler.
P.P.S. If anyone in silver/gold wants to test out my theory of the shuffle to platinum, I'd be happy to practice with them - I need to practice my shuffle-destruct techniques anyways.