The main reason why I can't display my sketches is
that they normally cost a fraction of the actual
frame.  There are some gorgeous ones that I would
really love to frame.  However, since it is so
expensive and I have very limited wall space, I
normally stick to cels that are at least 3 times
the value of the frame.

That is the primary reason.  Another reason is
that I initially started this hobby as a cel
collector and I am still stuck in the mentality
that cels are better than sketches because they
are the "finished product".  So, I feel awkward
putting them in frames.  

Another reason, as you say, is that they are not
as fragile as cels.  So, I can just slip them in a
UV slip/bag and just stick them on the wall.

SP