The "X-blade" that shattered in BBS was merely a prototype forged by an imperfect, experimental method Xehanort choose to try in order to see if he can take a shortcut (result: He can't).My head hurts from all the arguments on here. Maybe the light didn't because in BBS, Xehanort and Maleficent already was tricking Terra to collect hearts of princesses before the x-blade shattered. Xehanort already has intended to capture the PoH, even without the princesses having shards of light in there hearts or else after events of KH1, Xehanort would STILL try to capture the PoH because if they would have the 7 shards of light, they would be any important part of forging the x-blade. But seeing Xehanort hasn't been going after the PoH anymore, then they don't have further use. Xehanort said, "You have served your purpose, but now it's over." They were only used to open the final keyhole in Hollow Bastion for Xehanort to reach to Kingdom Hearts. But Idk! Nomura, help us!
On top of that this prototype was also highly unstable to begin with because Ventus mentally resisted the fusion leaving it unfinished and broken, as Vanitas himself points out.
The X-blade that is talked about when we speak about 7 light and 13 dark "shards" is the original X-blade, the one that shattered at the end of the first Keyblade War centuries/millenniums ago at the near-Apocalypse that ended the age of fairy tales.
Xehanort is not hunting those original shards but preparing current hearts as stand-ins in order to facilitate the required high dimensional clash to forge an X-blade, theorizing that by using the actual amount of parts (NOT the original parts) of the original it will result in a stable and "perfect" version of the weapon this time around unlike the version in BBS were he used only two parts and hoped that their purity would make up for the lesser numbers.
It just happens that for the light "parts" both the Guardians of Light and Princesses of Heart can be used as ingredients. It doesn't matter which group is used as long as there are exactly seven. It's even possible that a mix (i.e. three Guardians and four Princesses and so forth) would do the trick as long as the formula is maintained.
It's like cooking a soup that needs exactly seven carrots and thirteen pieces of meat. You do not need seven specific carrots, they just need to be actual carrots.
Thus, the seven light shards of the X-blade are NOT the Princesses of Heart.
The Princesses of Heart are the substitute for the lost original Kingdom Hearts, to which the X-blade was originally the counterpart.
Yen Sid states it explicitly that the Princesses are the source of all light in the current state of the universe => meaning they collectively take the role the original KIngdom Hearts had in the age of Fairy Tales, before the Keyblade War wrecked everything.