I know what you mean Vish.
I have to say if I followed that rule i'd be eatting 180g of protein a day.
Realistically I usually manage more like half that and I'm still building muscle.
I try not to have more than one shake a day cos I find they don't agree with me if I have too many. But if you've got a hardier stomach than me don't see why two or even three would be a bad thing. Must drink lots of water though as Stikfigure said, or it puts too much strain on your kidneys.
and don't forget the most important time to have a shake is post work out!