having good or bad alliances, betrayals and their in-game justificitions are one thing, while completely unnatural course of events the other.
if you have long alliance with realm A, but they leave you first moment when realm B attacks you, that is already betrayal which needs all kind of in-game justifications and politics.
if, however, realm A not only breaks the alliance but declare war on you, and ally with your new enemy, all within several turns, that is only tasteless play with buttons, such things cannot pass any test of rp-ing sense. not to mention that many even don't care to seek for some justification, couple of mumbling sentences and pressing the buttons is all.
that reduces depth of diplomatic play and depth of game in general. i doubt any medieval background could be found for blitzkrieg wars and blitz realm change of loyalties.
While quick changes of loyalties on level of single noble or even the largest city could have lot of sense (though it is seen very rarely) I see none for realm diplomatic buttons acrobatics.
