Alliances don't mean much.Most times when you share borders with an allied faction you WILL get attacked.There are of course some exceptions :)
Gauls won't attack you if you start as Casse. Eipiros won't attack you if you play as KH and make peace with them.Pahlava won't attack you if you play Baktria (and vice versa) etc, etc... I mean at some point they're BOUND to attack you.But not during the first 20-30 years or so... also allied nations will not actively help you when fighting a common enemy.Sometimes they may tie up some enemy troops in a far away front , othertimes your ally and your enemy will simply ignore eachother...
TRADE RIGHTS (with bordering nations) on the other hand are really important because they can jack up your economy (combined with ports etc)...