Chatting is especially useful when trying to conduct guild business (usually as an admin/prominent member) OR discussing in-depth strategy with a friend.
With instachatting, your also able to do unique events like TCA Yahtzee Game #1 which utilizes the "guild chat" option to quickly exchange messages. It'll make sense when you scroll down and read the rules.
UR Instachatting would be incredibly useful if we didn't have to utilize meebo or xat to hold guild meetings. Chatting one-on-one is good, but being able to chat with the rest of your admin/friends about UR business would greatly benefit any guild trying to better themselves.
Short answer: Having instachat on the iphone would be great. Multichatting from the iphone would be even better (without having to use non-UR sites). Maybe one day UR will release an update allowing us to do this. We'll just have to wait and see