Witnesses having full control over HBD interest rate, changing it on a whim, is not a problem. Note that witnesses control a lot more sensitive things, security and viability of whole chain rests on their ability to properly understand, select and manage changes. It is their duty to consider impact of their decisions on different [classes of] actors on the chain and it is our duty as voters to keep them in check and vote for someone else if we disagree with their decision.
I only have a problem with HBD in how ridiculously (from technical and usability standpoint) its interest payments are implemented. It should just replicate the VESTS model (effectively if not explicitly introducing VHBD as an asset held in savings balance) - much cheaper to hold (we would only need balance, all the seconds, seconds last update and last interest payment variables could be dropped) and also we'd no longer need to "stir the balance" once a month to get the interest. I wanted to change it long ago, but my lone voice is apparently not enough :o)
The claiming of the interest is a bit wonky I agree. But still not that much ofa pain, as most users just go woth jt. I would like it to be more than once per month, thats for sure.