I admit I'm rather bored by the idea of seeing Europes crowned heads at the wedding. Infact, I see it to be more of a PR 'exercise', though I understand that any invitation from the Court of St James should first and foremost be extended to it's continental affiliations.
However, it clearly suggests that even the heir to the heir apparent (be it in the name of the Queen) can muster Europes ruling elite. Yet again, the House of Windsor reigns supreme in the preferential stakes as their relations and counterparts seem only too eager to attend at the highest level. A courtesy which is selectively extended by the Windsor's.
I'd have much rather seen the heir's apparent and their respective spouses attend in representation of each sovereign.