I really don't know. The only reason I can think of is that when someone wanted to whisper them and they were not around to click on it would be difficult or even close to impossible because the person would not know how to type the fancy letter/symbols in their name.
This happened to me with a close friend once and it was always a pain trying to talk to her because I hate the mailbox and prefer whisper but since I couldn't type her name it was impossible unless her alt was near me .-.