I've been a digital scrapbooker for almost 15 years now, so I definitely delete the old stuff. Styles have changed and design skills are so much better now compared to when we were all first figuring things out. LOL
For templates, I keep just one of the layered files and delete all the rest in the zip (.page files are really large and I don't use them, for example). I almost never use a template twice, unless it's something very special, so once I have pages printed and in a book, I delete templates I've used. I keep mine organized by number of photo slots, so a 4-pack of templates isn't kept all together most of the time anyway.
I tend to only purge older kits or in styles I don't use anymore. I'll pull out the unique pieces of a kit I want to purge and store them in folders for such things (such as hand-drawn elements or unique ephemera I can use on any page).
I enjoy the organizational process, but not frequently. I tend to purge once a year.