Yeah, there's not much a person can buy with one Hive currently I think, but down the track that could, and probably will, change which is why I'm holding all of mine; it's also nice to be able to vote on others with a little more stake, reward comments and all.
I'm not sure about the interest thing and what affects it but the more you curate the more you'll earn and stake helps that along. I usually upvote daily down to about 80% of my HP, sometimes down to 77%, and if I get to the end of the day with HP left over (over 80% of my HP) I will drop it on someone's post of comments as I have with you in the past. It keeps the curation rewards coming and still allows me the chance to add larger votes with my HP (like I have with your comment here) rather than miniscule votes.
I feel people tend to be quite frugal with upvotes but there's good curation to earn, on comments also, and I can't see that as a bad thing. It also helps others out. It's amazing how thankful a person can be, and has been, due to a simple upvote and comment I've left them. it's amazing encouragement. Of course, I won't support those who don't do regular power ups and simply extract all their earnings.