If I have a powerful but far-off ally who asks me to join their war at a moment when I'm too busy with my own wars, is there any disadvantage to accepting their call for help but then never actually sending troops to help? Will my ally be offended in any way? Will they be more pleased if I actually do send help?
I know that there are certain intrigue options that become unavailable if you're at war, but let's assume that I'm already at war and ignore that aspect...
As far as I can tell, the only opinion boost you receive is when you say yes. Your actual "contribution" is only relevant for prestige.
"However, you receive the opinion bonus and avoid the prestige hit for "joining the war" whether or not you contribute to the war or even raise your levies." (Found Here)
The one thing you need to be careful of though, especially if you're fighting another war and need your troops to win that one, is if the person you're joining the war against is nearby and much more powerful than you. If they're not either, you're probably fine.
There is no prestige or opinion penalty to contributing 0% to an ally's war.
The main disadvantage is that joining an ally's war counts as being at war. This stops you changing succession laws or making decisions that require you to be at peace such as holding feasts. This can be problematic if the war drags on- as an ally there's not much you can do about it.
Also of course the issue that they could start sieging you and hold your land!
