"It's not what it looks like that's important, but what it can do."
This has been an attitude which has bedevilled RISC OS ever since its inception. Like it or not, the way an OS and its applications look does matter to people. I remember in the 90s, when RISC OS was arguably much more fully-featured than Windows 3.1, marvelling that Microsoft's products looked so professional. They may have been clunky to use and much less capable than many RISC OS alternatives, but to the average user it didn't look that way at all. Similarly, Apple have recently made the running in the GUI stakes - their software just looks so good and, as a result, people use it.
If you like the NewLook style, then that's fine: the Theme Manager doesn't take it away. But providing choice and opening up the way for more modern designs seems to me a worthwhile thing to do. And, as epistaxsis says, there is no real performance issue in swapping desktop sprites over: fancy transparency effects and other tricks is, perhaps fortunately, not going to grace RISC OS desktops for some time.