Such a nice thread as I've been wondering that myself. I want a Zeiss but I can't decide between the 50, 35, or 28, and I can only afford one if I sell my current 35. Last year I was a 50 guy... this year I found myself gravitating towards the 35... and I've recently discovered the joy of a 28.
I dont think I'd be able to live with just one focal length! There are times where I find the 35 a tad too wide and myself unable to get closer, so I might just have to go with the 50.