Like many convertibles today, I'd imagine that phaetons are quite desirable, although maybe a bit less practical for some. Browsing through Hemmings and eBay there are usually quite a few available so you wouldn't think that they are rare.
As mentioned earlier, it could just be that they are less desirable for touring. At the regional and national meets there are usually only 5-6 phaetons out of the 200-400 cars present.
We have thoroughly enjoyed touring with our phaeton, lots of fun! It does have its drawbacks though. Like the lack of being able to secure our belongings when we stop at points of interest. We usually have to pack our stuff with us, or bum some space in the closed cars with locking doors. Wet weather also provides for some interesting drives, we usually arrive somewhat wet, but those drives have become some of the most fun and memorable.