Lets see... Usefull for learning strings, Useless for using it. Why useless? Well : 1.When using /, it will be recognized as a Battle.net command to whisper other online players. 2. This is totaly useless, whenever a player is setting he's chat to allies u got the same effect and no double words. Meanwhile for talking to all u can easly press shift + enter to talk to all. P.S : When setting to observer, it doesn't have any sense at all, the text is just repeated..
Edit : Use \ for whispering , just came with the idea up.