I asked in their discord for the same thing and the explanation that they gave me is :

as hive is so low in price they have to spread their voting power, so if you boost your post with more than 1.33$ it have a lot of chance to not be upvoted, they just dont have enought voting power to give high amount voting to everyone so their advice was to keep the boost at 1$ or 1.33 ...

for me those pooints are worth, is a nice way to get at least 1$+ in my post, those are good for long term