I liked the Bolas explanation. It got me thinking of something to add to it.Flavor-wise, players are planeswalkers. And we're more powerful ones than those represented on cards. We have 20 life, rather than single-digit loyalty. We have a little more than ten spells to sling (going by 36 non-land cards in a deck and the idea that some are four-ofs but not all), rather than three (or in the case of Jace, the Broken, four), though it varies.So if Bolas (and other villainous 'walkers) are interested in collecting: there's a trophy for ya, someone who has pre-Mending levels of power. And if you're not Bolas-level powerful but are an opportunist... well, a player would be a very, very powerful ally, even if s/he loses.And hey, if Bolas can have lesser planeswalkers as minions, the idea that the planeswalkers in, say, a black/blue deck are minions who may or may not be happy about this arrangement but come when called makes perfect flavor sense, too. (As does their skedaddling when they're out of loyalty. It's so hard to find good help these days... )
That doesn't quite work. First of all the loyalty don't represent a planeswalker's toughness (or life total). It more or less represents Loyalty, to you, the planeswalker that summoned them. And the "spells" they're able to use doesn't represent the full extent of their power. Or do you really think the ancient dragon Nicol Bolas is only able to cast 3 spells? This is how there can be multiple versions of a single planeswalker. Jace is a prime example. Every Jace-card represents Jace, just with different spells. And then there are the signature spells in the core sets. And in the books Jace is able to cast even more spells. And that's not even mentioning his duel deck. He seems to have the arsenal of a dedicated mono-blue player, with a couple of splashes of his allied colors.
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