I loved the new design of your site, hats off!
I didn't have any spare battery for my Desire S, but I did change the stock one after a year of usage after it wore out and I saw a definite battery life increase (I did get a slightly bigger one though). So it's the only real use of a removable battery feature in a phone I see. On my Nexus 5, if the battery gets old, I sadly can't swap it, though it won't affect my user experience too much.
About fingerprint scanner and heartbeat sensors:
I think about both of them as gimmicks. I mean, as you guys, I haven't seen anyone actually using TouchId on their iPhones and I don't think people will ever use heartbeat senors to check their heartbeat. There might be people needing it, but then again, they could go and get special devices for that. And I do use face unlock to make my iPhone friends jealous sometimes, it really works. :P