I would think adding wifi will be good enough for most international users.
This is something that very few, if any other companies can claim. Not even cell phone companies can claim this - Verizon needs AT&T or Sprint or whoever in order to allow people to talk to everyone else.
Adding wifi support removes this claim - you're now dependent on the person's ISP (internet connection), router manufacturer (provides the wifi) as well as a few other misc things. That takes away from the simplicity of the device.
If the EVDO connection is available in their area, great. Use that. But if it is not, users can figure out how to set up the wifi connection. They have to figure out how to set up the wifi connection in their notebooks anyway.