This is a workflow thing that I know I had to adjust to. Any changes that are made in setup mode will affect all of your animations. My advice is to set things up in set up mode (as the name implies) and test things like weights and binding in an animation. It may take a little to get used to the back and forth but you'll get used to it in no time.

Hi Krblshna, if you take a look at the example projects that come in the Spine installation folder, you'll notice that the most common setup pose they have is an idle position.
For the way Spine works, every animation is relative to the setup pose, so if you move a bone in setup pose, it will reflect also in all of your animations. This is great when you need to add new gear, or slightly change the posture of your character, etc. but as Puppy said, it's something that requires a bit of getting used to. A good habit you need to start having is, as Puppy suggested, to test your rig and poses in animate mode, and make the needed adjustments in Setup mode.

Regarding multi-pose character, you might find this series useful: Spine Twitch Streams - YouTube

It revolves around reusing the same bones to attach different angles of the same body parts. An addition to that is that you can also place your images in setup pose, turn them into meshes, and then update the bindings during animate mode to get the new pose correctly.
You can see an example of this workflow around 2:47:40 of this video:

Hello,
I've been encountering this problem, and I think I found one situation in which the suggested workflow doesn't work very well, but with an easy fix.
Since constraint priority can't be changed in animation mode, having a lot of constraints and trying to figure out the correct order to apply them in is a pain.
An easy fix would be to enable reordering constraints in animate mode and have the changes reflected in setup mode. This would be similar to how you can adjust vertex weights in animate mode.

Edit: changing any constraint property (local, relative, compress, stretch, uniform) is quite a pain with the current workflow.

Erikari had the idea to use Preview so you can adjust your constraints in setup mode while seeing the results for an animation.

Many things are not allowed in animate mode solely to separate the setup pose from the animate pose. You don't want to accidentally mess up your setup pose and not realize it until later. We do allow changing the setup pose from animate mode in cases where that is convenient, like weights. I think it is OK if we allow changing the constraint order in animate mode. I'm not so sure about the other constraint properties you listed. When do you have trouble with those?

Preview is better than nothing, but it's not the same as being able to manipulate bones directly to check that constraint settings are having the effects you want. I suppose setting up a few test animations to check certain cases constraints might not act properly in could help.

It's probably because I'm still learning about what some of the constraint properties do, but I use a lot of trial and error when setting up my constraints. I'll set constraint properties one way, drag around the target bone, set the constraint properties another way, etc...

Because it's a pain to have to switch to animate mode to test every time, I end up changing things in setup mode and then undoing the changes I made while testing. I inevitably forget to undo a change that was for testing.