- The main reason is that the editor is where it's useful. InstanceIDs allow editor code to associate objects at edit-time with objects at run-time, even as they are destroyed and recreated. There's no corresponding need in the player.
- There's nothing that specific. GameObject.Find, GameObject.FindWithTag, and Object.FindObjectOfType have related functionality. They're all pretty slow, like InstanceIDtoObject is.
Object.FindObjectsOfType(typeof(GameObject)) will do it, but again, it's plenty slow.
There's a lot of different ways to find objects. Other than the slow brittle last-resort ones I mentioned above, there's having a reference to one (either set per-object or in the script's import options) or having the object register itself during OnEnable. Which one's the best is heavily dependent on your particular need..... what are you trying to find, and what are your constraints?
Aug 07 '11 at 07:43 PM