There's that, of course, or he could blackmail the Tenryuubito if he ever gets his hands on Joy Boy. His goal is to 'destroy every last part of the world they (the tenryuubito) control'. If Joy Boy's existence is revealed, who knows how people would react.I doubt that Joyboy would just reveal important information like that to someone he doesn't even know. If anything he would realize that Doflamingo is just as corrupt as every other Celestial Dragon and refuse to tell him the location of the AWs and Raftel.
"I'd have been able to use the Treasure of Marijois and hold the reins of the world!"
'Holding the reins of the world' is a figure of speech. Just like when you're holding the reins of a horse, you're controlling it and decides where it should go. Therefore we can (perhaps) assume that his goal is to control the World Government behind the shadows, blackmailing them if they don't do as he says. They, the government, would not want someone to reveal the existence of the national treasure at all cost, it seems.