Users must be able to use 'native' look and feel.
Spark uses the 'native' look and feel only when no default look and feel has been specified, and no preference has been provided to the user. Once a preference has been selected, there is no way to go back to the native look and feel.
It should be possible for users to, at any point, use the 'native' look and feel, instead of a previously selected other look and feel.
When you make this option available, i will check how it looks on Win\Linux (will probably ask speedy to check on Mac) and will decide on whether to change the default to Native.