CSS3 has been around for a long time and believe it or not, W3C published an article Introduction to CSS3 in 2001 (9 years back!!). Firefox and Safari users have benefited from CSS3's new options for borders. But as many web developers have realised, there is a need to ensure the technique to produce rounded corners must work in all browsers.
Internet Explorer still holds a strong share of the market, mainly due to the corporate and school policies. IE6 will live on for another few more years (yes, it's a pain). While we enthusiastically want to adopt the latest technologies and showcase them to our audience, we remember a big portion of our audience are not using the latest modern browser yet. A camp of developers might argue that it is certainly possible to detect which browser the user uses, and the website will react and pick the right technique to render the box. CSS3.info reports that CSS3 is not ready for cross browser wide adaptation yet. Your stand?
Even if all browsers support CSS3 in the future, TBP believes that the native border options available will not be able to build fanciful and more complex boxes, and they will only help in the most basic boxes. Since there are many sites which need more than simple boxes, TBP's technique will certainly stand out through the years.
