Ye I agree that helping an ally should be much more rewarding then it is now and it is indeed too easy to just not send any troops at all after you accepted the call for arms, although I don't neccesarily think it should be patched, as helping an ally is for me more of a role-playing feature, if you are just a jerk you don't help your ally, if you like to be a honourable guy, you will help you ally till the end.Gaining something from helping an ally
So far, when you are called into war, you only have the choice of:
a) losing some prestige and declining
b) not losing prestige and accepting
To make people actually take part in the war they're called to, I suggest making an event which would give some prestige (and maybe piety) when your ally wins.
I see this as a minor change. If it's not doable by events, then, well, I think we'll have to wait for this feature for 1.08.
And I agree with the OP. All these fixes should be in for 1.07c.
Even though I think it's not neccesarily game breaking, here a some fixes that could be added to make being a ally more rewarding.
Minor fix would be: Double the amount of warscorepoints you get as an ally, so you can get more of the prestige reward that you get now.
Major fix: Make ALL rewards share-able with the allies(except land). ALL the money, prestige and piety should be completely shared among the allies when you have won. Combine this with extra warscore points as an ally and it really might be worth it to be an ally. Also, You should not take a prestige hit for refusing an offensive war for land(you will still lose prestige for refusing to help in civil wars).