So obviously since the rates are abysmal, not everyone's going to get two extra Marquis de Leon for that nice STMR, but just in case you're among the lucky ones, it'd be in your best interest to wait before you merge them.

The reason for this has been mentioned before but I'd figure I'd say it again just in case. Let's say you have four of the same unit; you get the most out of your dupes by first getting two of them to 7* (since the amount of 7s* you've awakened can net you precious extra UoC tickets) and then fusing both units together for an STMR and an extra 50% moogle. However, if you happen to have a 5th dupe, you could fuse that unit to one of the 7s* so that you get the 50% moogle, then fuse that 7* to the other one for another 50%, earning you a 100% moogle.

Now, if you save those Marquis until the end of the Raid period and use up all your coins, there's always the very small chance you could get that extra unit that can get you the 100% moogle, and if you've already fused four of them you'd be left with a lone Marquis that will probably serve no purpose whatsoever. Since this is a limited unit with little to no use other than merging and getting that STMR and the 7* counter up, it seems worthwile to wait just a little longer to make absolutely sure you've gotten as many as possible. I think the very small chance of an extra 50% moogle is totally worth it.