I own a 5G 30GB iPod, and got a used shuffle on the cheap for many of the reasons people have posted. It's easy to use, you don't have to look at it to use it.
The iPod click wheel is the best interface ever, but you have to be careful when you reach for your iPod, so you don't mess up the volume, etc. I understand you can use the hold switch, but it can all be a hassle if you're doing something while listening to music.
With the Shuffle, you can feel for the buttons and pause/skip/etc. without looking at it. The lanyard is also handy, and you don't have to worry about scratching it up, etc. Finally, my listening habits actually change for the better, because I have set up a bunch of smart lists that I randomly autofill on my Shuffle. (of course I can basically do the same with my regular iPod, but the autofill feature is easy to use and for the Shuffle only.)
I'd consider a cheap flash unit from another company, but will take the Shuffle's great iTunes integration over a flash player with a display from another company.
The nano will continue to increase in capacity, so there should always be room for a cheap Apple player. If anything, maybe Apple can come out with a new and improved Shuffle with a small 1-line display or something.