i guess it's obvious why sketches are gaining
popularity, CG series don't have cels so sketches
is the only original artwork you can collect.

As for me i prefer cels than sketches, but of
course i try when possible to get cels that come
with it's matching sketch. Until now i have never
purchased an standalone sketch except once not
long ago because it was an sketch of a cel that i
already owned.....

I'm still trying to wonder which level of success
rilezu (post-production) may have in the near
future, they many times come with the sketch that
was used on the series, and only one of each is
produced.... but still it's not the same as a
production cel....

Anyway, i have heard that sketches won't be needed
either on the future (maybe only rough gengas will
be needed to make a concept of th character)...
but of course i'm not sure if this is true or not.