I think it's important to remember SWT came in to being when Swing was very immature and I guess the Eclipse folks thought it necessary to reinvent-the-wheel (suspect in retrospect that's something they regret, however thin a layer SWT is).
Nowadays Swing has matured, many of the problems and performance issues it used to have have been fixed and with the 3rd party extensions like JGoodies and JIDE I can't think of a good reason you would use anything but Swing for thick-client development.
