User blog comment:AYET/The Mera Mera no Mi and Doflamingo/@comment-4605926-20130303210239

Don't forget that the fruit doesn't make the user powerful, the user makes the fruit powerful. The Mera Mera no Mi was as powerful as it was in Ace's possession because Ace was powerful even without it. I don't think that giving the Mera Mera no Mi to any crew member or ally will create such a drastic significance in power that it would be worth eating so quickly before finding out a better use for it. Doflamingo probably has many business partners, leads an underworld organization and his own personal crew and is allied with many different New World pirate crews. One fruit wouldn't seem to make a real difference in such a large group. Furthermore, as Doflamingo is highly affiliated with the underworld and is a major figure there, it's only logical that he would manage to get his hands on the Mera Mera no Mi. Also, we weren't told how recently he found the fruit, it might have been not too long ago which would then solve your question as to why he didn't put the fruit to use until now.

I will now refer to each of your theories individually:

1) I do believe that this is the case of sorts, but unlike you, I'm not the least bit surprised that the fruit isn't of any great significance in terms of power to Doflamingo's whole organization might. In fact, it seems to me pretty much straightforward that he would want to use it for a different purpose. Even if it does get eaten, it would probably only be accidentally, by a really greedy ally of Doflamingo, or much later until its purpose has been fulfilled. I'm not even sure how beneficial it would be under the Straw Hat Pirates' control. I can currently see no member it would really fit for. Sanji and Zoro are the swimmers, and the crew needs swimmers for the devil fruit users. Franky already has more than enough fire power if you ask me. It might fit Usopp but he is too wimpy and weak to put it to good enough use in my opinion. Really can't see Nami using this fruit...

2) Of course he has a grudge against Luffy, Luffy's alliance is planning to sabotage his business and who knows what else.

Now that I think about it, the Straw Hats are in deeper than anyone could imagine. They are currently targeted by Fujitora (a marine admiral?), enemies of Doflamingo and all who are affiliated with him which includes Kaido the Beast, enemies of the Big Mom pirates and its respective allies (unless of course Charlotte turns out to be Lola's mother... Another point for thought). They have one hell of an objective, how they're going to accomplish it is truly beyond me.

In conclusion, the Mera Mera no Mi is better off used as a strategic item rather than a powerup. The alliance is indeed in deep s**t. Oda wants to mind f**k us in this whole New World business which just seems to get more and more complicated chapter by chapter. As we solve mysteries new ones hurry to replace them. That's why I love One Piece so much! The only series that managed to keep me hooked for so long without getting bored in the process and the only one that doesn't mind f**k you logically (which in case you haven't noticed is very important to me).