The discussion on this answer to the question \"How can I use Google\'s new imageless button?\" Has prompted this question.
Google seems to think that going imageless i
The page load is smoother as no images have to be loaded and will appear later than the rest
The button text is also readable in the case somebody cannot read/view images, yet you have the graphical look. (building a graphical button with images in the traditional way around real text is as complicated HTML as this method)
As they mentioned in their blog, these buttons are skinnable without creating and storing custom images.
Basically, you get all the advantages of plain text buttons over custom imaging, while still having a nice, skinnable graphical look.