Sometimes it comes down to whether user-generated content with externally linked resources is allowed on the site. E.g. forums often allows embedding images by users and because users don't care about HTTP or HTTPS and only copy the image link from somewhere, you will end up with "mixed content" warnings and broken "locks" in the browser address bar all over the place.

The current alternative is to simply use HTTP, which doesn't yield any warnings. When Chrome will mark all non-HTTPS sites as insecure this will change, hopefully.