Although they did overdo it with the amount you can get, I'd say this is one of the Fire Emblems -if not the- where you need it the most.
I disagree. If you can find a way to kill Thieves chasing/obtaining treasure before they can leave the map, you can usually manage. Pegasus Knights are helpful in this task on large, open maps like Chapter 9 or the chapter which you encounter Garnef (and even then, I can kill the Thief without a Pegasus). Thieves never take more than one treasure, so you can still kill them and get the treasure if they open a treasure box..
This is why I find the warp staff so cheap, because using it isn't a requirement, so you can save it for doing things like clearing Chapters in a single turn and whatnot.
And I do agree that the Rescue Staff is much more necessary in Book 2 than the Warp Staff is. By the time you get the Warp Staff, I think you also are getting the Thief staff you can use to open chests, allowing you to use the Warp Staff for other purposes (and I think you still get more than one).