But PoHs being a special existance, and also carrying the power to open the Door to Darkness make them have a really important role.
A power which is passive by nature. Wielding a Keyblade is active and by definition requires a strong Heart. You also get to actually do something with it, and with Sora losing it, the Wielder's state comes into play.
As long as all the PoHs are together at the same place you'll get the Keyhole.
It doesn't seems like someone with a weak heart could be qualified to be a PoH. At least not for me.
Again, Purity of Heart =\= Strength of Heart. Again, it was never hinted even once to a PoH having a strong Heart, and the best explanation we have as to why they even
are PoHs is because they were
born like it. It wasn't even that the Princesses were described as being strong enough to ward off any and all Darkness - they are
incapable of housing Darkness, thus it doesn't even matter if they're weak Hearted - they can still fulfill that role.
Kairi
losing her Heart at the start of KH1 always seemed to be proof to me of her weakness, but because she had the PoH status serving as a "God mode" cheat, she didn't cast off a Nobody on her own
that we know of as long as Xion isn't confirmed to be it and managed to hold the Keyblade Riku gave her - which could be seen as Riku willing and nothing more to it if we want to go there, which I often do.
Or maybe I'm just searching for more reasons to hate Kairi.
Oh, sweetie, don't get me wrong, I'm hardly protecting her.
I'm stating a fact saying Kairi is ironically probably the person with the weakest Heart around. XD