Size. Using styles like this lead to a small decrease in page size, which over time on a very high traffic site leads to a notable bandwidth savings.

... or at least, that was the argument. In practice, most teams didn't adopt it unless mandated from above; because it sucks to use, has an infinitesimal decrease in individual page load time, and the bandwidth savings certainly don't outweigh the lost developer time.

But hey, all you gotta do is convince some VP that your niche approach is the right one, and you'll get some adoption in a large company like Yahoo.