Encrypting only certain pages can be confusing to visitors, who may not understand why some pages are secure while others aren’t, and it may raise concerns about the overall security of your site. A fully secure site makes it nearly impossible for bad actors to impersonate your site or shady advertisers to insert ads (or worse) into your visitor’s connection while they’re on your site (a particular problem at hotel and public hotspots). It’s technically easier to encrypt everything and it requires less ongoing administration.