Why use Tiff? No reason not to. But I haven't given up on PSD quite yet. Because:
-big 16-bit PSD files open and save much faster than Zip Tiffs. (Granted, I haven't tested this in a while, but when I did the difference was significant.) With the fast processors and gobs of RAM available these days, the opening and saving of files has become, by elimination, my biggest processing bottleneck.
-when I see a PSD file in a folder on my computer, I know that it's the main, full resolution, working version of a photo. Of course, that has nothing to do with the quality of the format. It's just part of a system that evolved as an easy shorthand for my purposes. (Raw files in one folder called, aptly, "Raw." PSD working files in another folder called, "PSD." Final TIFFs for delivery in a third folder called, "to deliver.")
-when the Photoshop apocalypse is imminent, I'll do what Ctein suggests. Until then, I'm not sure there's a big hurry. When I installed Lightroom, I had to re-save all my "un-maximized" Photoshop files to satisfy its limitations, and that was a frustrating pain in the neck, but I got through it. Now I have Lightroom itself, and other tools, to make the job pretty easy.
Long story short, I don't think file format choice has to be an all-or-nothing decision, and not one I have to make today.