Traditionally https was only used when absolutely necessary to protect personal and vital information. Checkout and account registration being the most common. The major reason for a lack of broad implementation was the computational overhead that slowed down performance. With the computational overhead no longer an issue(if you don't think so please research yourself and test, test, test), what is the proper use in today's environment.
As I have been working on my sites I have started to come to the conclusion there is absolutely no downside to using it on the entire site for the following summarized reasons.
1) A good EV cert enhances customer confidence (a green address bar is never a bad thing)
2) Search engines still find your products (submitted through feeds anyway)
3) Performance is not an issue any longer
4) If you pay for the cert you may as well use it and get the enhanced credibility
Is my logic flawed? Greater minds than I are on this site, looking forward to feedback.