Hansels get very large ally bonus on attacks, but very low base plunder. So unless you only use your spies, yes allies are important.
There is no ally bonus when assassinating. But even for hansels they need to attack, hence allies will give you ally bonus.
Your allies will of course contribute to your ally bonus plunder as well as your end bonus for attacks (you get around 50% of your immediate hit payout as endbonus). If you are a pure spy (or play as if you were one...) you won't need any allies.