Well, a lot of publishers use the page-template badly, putting margins in it that should be specified in the css. In fact it seems a lot of them just paste in a single page-template file that they've found somewhere in the mistaken belief that it makes their output look more professional, but that's a symptom of the poor level of coding skills prevalent in the industry.
Used properly, the page-template extension offers a reasonable amount of extra functionality. It's non-standard, but we're already seeing people playing around
with webkit-specific css3 elements to get nice effects on the iPad, so the epub standard is in danger of losing relevance.