Fancels are done by artists other than the studio 
design team.
The test cels are done by studio designers to 
check to see if color and design stack up.
Its not as if some third party is doing them.

They are like the Eva-hand painted repro cels but 
just fewer of them exist and the Eva cels are 
POST production. These are pre-production cels. I 
view it as to paying for a preliminary character 
design sketch. Ie a fascinating look at the 
design process. 

Look at the X-Manga
Look at the X2 Music Video
Look at the X-Movie
Look at the X-TV character designs

Other than the manga the 3 animated versions have 
been completed by Studio Madhouse over the past 
decade. And its is interesting to see the process 
Madhouse has used - the 
X2 characters are much closer to Clamp's original 
designs - more elegant looking than the movie - 
progressively becoming simpler and more "solid" 
by X-Tv.

If you think people are nuts buying them. That is 
your perogative. Plenty of people think cel 
collectors in general are nuts anyway.