if you have 15% APR on HBD saving would you still hold HP that could give you 15% if you are lucky with your votes?

There are a number of tradeoffs between HBD and HP, not just comparative yield. Mostly that HBD offers stability and greater liquidity, but limited upside and HP has limited liquidity but more upside. How those play out in terms of demand is really anyone's guess but it isn't hard to believe that HBD might offer higher yield, but people still want to invest in HP for the chance of (potentially large) price apprecation.

Now historically Hive/Steem has not done very well in terms of overall price appreciation but I guess anyone investing in it must think that is likely to change going forward.