I'd rather get more 5 & 6* shards than get shards for limited time cav crystals. If you're extremely lucky then you can get multiple 6* champs, but if not the chances are way too high to only end up with 3 & 4*s. There's already such a high chance of pulling a useless champ in a 6* basic, I don't like having another layer of RNG with star value.