It's not that surprising at all that Kabuto's managed to get a hold of these particular shinobi (including the Kages), considering Akatsuki played a DIRECT role in many of their deaths.
Just take a LOOK at them:
http://ani-haven.net/hr-alpha/Naruto/516/18
Akatsuki (or Orochimaru once he left Akatsuki) killed/controlled several of those shinobi. In addition, Kabuto said earlier he had several failures when building his collection, so clearly he wanted way more than he ended up with.
It's true the manga stated early on villages made a point of disposing of bodies to seal their techniques, but that means different things for different villages. Consider Neji's father, who had a symbol tattooed on his forehead that would seal his Byakugan upon his death. Well, his DNA is still ripe for the taking even if the Byakugan is gone. I'm betting several other villages have a similar seal sort technique allowing them to bury their dead "safely", but never counted on a rare technique like Edo Tensei (only THREE people in the entire manga can use it...THREE).
Anyway, I've got no problem with Kabuto "spamming" Edo Tensei. It's clearly got some major draw back Kabuto hasn't revealed (smartly so), and almost no one else can use it.