A UI is as it says an interface to interact with the software.
Having special effects doesn't necessarily mean just cheap thrills. If its done with tought and good design, it actually enhances the user experience - not in the "Wow that is cool sliding effect", but in reducing the amount of time it takes for an average user to interact usefully to do a task.

Or it increases the choices in how you can accomplish something.

Please don't equate UI effects to only graphical gee-whiz effects. Its really more than that.

Claws having multiple panes unlike many other apps is itself an UI effect so to speak. Similarly a sliding window to reveal additional info is also a effective UI design. Just because it slides gracefully doesn't mean its of no use.