I'm probably a little different than most people.
The main reason I frame a cel is size/protection.
I have a number of cels that are too big for the
largest Itoya portfolios. I felt it was the only
way to keep them from getting damaged. Otherwise
they would just sit flat in the same drawer that
I keep my skteches in. I'm sure the weight of so
many multilayered large cels would harm the ones
on the bottom. Framing based only on size has its
drawbacks. Sometimes I spent more money on the
framing job (I don't trust my own abilities) than
I did on the cel.
I have thought about framing a few favorites,
since a frame can really enhance a cel's
appearance, but I haven't done it yet.