User blog comment:AYET/The Mera Mera no Mi and Doflamingo/@comment-3119203-20130301054842

Well, logias are exactly fun to deal with, even when you have haki. A logia on the level of the Mera Mera no Mi, which was used to fight admirals and Shichibukai, is especially dangerous. If I was Doflamingo, I wouldn't want anyone, even my subordinates, to have that. Doflamingo already lost Law, maybe he's a bit paranoid.

In addition, as DP stated below, Doflamingo is using it as a bargaining chip, sort of a counter to Law and Luffy having Caesar. If Doflamingo can get Luffy to break from Law, Law's battle power will be greatly reduced. Law cannot stand up to Doflamingo and his three top-notch badasses, so he needs Caesar to serve as a decoy. Personally, I don't think Luffy is going to give a crap about the Mera Mera no Mi, but who knows....