Okay, so you know how it's really uncomfortable to talk to someone by Floo powdering your head? Why do they bother? Wouldn't it be easier to just send your whole self over and Floo powder yourself back? If Harry had done that when he was trying to talk to Sirius in book 5, he totally could have just gone over and found him in Grimmauld Place and then gone back wearing his Invisibility Cloak. Is there something in the book that made this impossible?