As far as getting the habit of frequently changing themes, you cannot help but wonder if they would be a cause for occasional slowdowns of computers whenever they are in use. One thing we can note is the fact that they eat resources.
Unlike in the early days when you can simply change your desktop with standard themes, the new themes we use in Windows XP and Vista need larger resources. They are enhancements for people who want their computer to look unique and ideal to their eyes each time that they use them. For others however, the only important thing is being able to use your computer and get the work you need done.
For some, it may not matter. New computer specifications today allow them to enjoy the same speed and output although it may defeat the purpose of being faster when it comes to processing and resources used. Some would point towards the way of new operating system needs that slow down a computer and adding new Windows themes only aggravates it as well.
With these things in mind, we cannot avoid using some themes at some point. Perhaps modifying them according to what we want or in part can help avoid such problems in the end. It may become technical for some people who are not used to customizing themes on their desktop but if you want to maximize computer performance, you can do so by picking only some of the elements needed to make your computer stand out and be different from the rest.