Perhaps I'm alone in this, but I would like to be able to have more control over the UI when designing a skin. For instance, I want to create a skin that uses extJS, but this is currently impossible since RoundCube outputs everything for me already formatted whereas I want it to only output the information or bare html so I can choose how to manipulate it.
I agree with yo that some objects might be more open to 'manipulation', I.E. the sequence of the displayed mail-headers
But the use of makes it very easy to create or modify a skin. Furthermore, if you have any ideas on how to add more flexibility, you might want to open a ticket at RoundCube Webmail ? Trac (http://trac.roundcube.net/).
I wanted the ability to add the column sequence to the skin, and opened a ticket and submitted the code. This code have been modified a bit and approved and is now part of RC 0.2-stable.