Lasting tourney bans have been evaluated after whether or not they ruin the theme of a tourney or not. That's always a risk with Other cards and some other ones that can avoid the theme and still be viable, such as Animate Weapon, Discord, Bolts, Poison, Dims and Miracle stalls. If there's a tourney theme that would survive having Shards, they'd be allowed, but frankly anything revolving Shards sounds incredibly boring as a tourney. We'd rather have strong elemental themes, as that's where the identity of the game and creative possibilities lie.
Noting, new rares are always banned for a month or two because we have to give people a chance to get them first.
And, of course, the Leagues' metagame is the only completely unrestricted one. Not something you see anywhere else, for good reason. It's a lot more common that shards are banned whenever someone speaks of "the meta" thanks to PvP Events, Tourneys and Wars. The bigger a game gets, the more restrictions do you need to create a challenging or creatively demanding and varying metagame.