That's not my experience. A resurrected AI brings back any and all modifiers you had before he was wiped out. So, you get an ally for life but ONLY if you already had good relations with him. Hardly much of a benefit. But again, that's something I think they should change. A resurrection should get you an ally for life... and currently does not.
I don't know many examples from real world history in which nation liberated by other nation would be its ally for thousands years only because it was "liberated" - your point of view is based on thinking "liberation = Freedom and Love, let's hug!". My homeland, Poland, during WWII was "liberated" from Third Reich by USSR. In Civs terms everything would look OK - good uncle Stalin resurrected Polish country! In fact, after extremely bad Polish - Soviet relations in 1920 - 1945, and devastating "liberation" by Soviet forces, Poland after few years was "independent country" - practically under Stalin's control, and today Polish - Russian relations are still bad... Something similar can be told about Spanish - Mesoamerican invasion which liberated many countries occupied by Aztec Empire, and they later realised that Spaniards are much bigger threat; or about many countries in renaissance Italian Wars, during which allying against former liberator was something normal.
Imagine you are the leader of Civ nation which was bullied by two foreign powers, and later one of these powers (with machiavellistic leader
) paid first one to attack you - only to liberate you later for diplomatic bonus
Why would you have to love "liberator" like this?
I don't see anything strange in the way liberation is shown in this game, for me it works perfectly - if I release nation with which I had good relations before, they love me. If they hated me, well, firstly why should I liberate my former enemy? And why should I expect him to love me, only because after 2000 years of bullying him I released his devastated country - only for my own political benefit? This leader can think "nice, that helped my nation, but now I must do something with total economical domination of my sweet liberator - for raison d'État!)
A resurrection should be useful for your empire, or turn friendly/neutral nation into "thankful ally", but I don't understand why it should change your worst historical enemy into instant lover. That's silly.