If u can articulate and convince urself that u need it for your photography, go for it.
Dun be put down by what others have to say. You can borrow / rent a copy of the 70-200 and work with it to see if it is what you would wan to work with.
The itch ? most of us have the itch, just how itchy and how badly u wanna stop the itch.
I learnt that I only buy if i gonan use it often. If not just rent when the occation calls for it