No, nothing crucial to the plot happens during either DLC. There's a tiny bit more development of Carolina in Memory Sequence 12, and arguably even LESS development of Machiavelli in Memory Sequence 13. Had they naturally occurred BEFORE Memory Sequence 14 for those of us that beat the game months ago, then perhaps their presence in MS14 would have been a little more believable, but as it is, these DLC Memory Sequences merely flesh out what the game tells you in MS14 in any event. So there's precious little "new" story, if you get what I mean.
That said, as much as MS12 was throw-away piece of shit example of DLC, I've got to say that MS13 was a massive improvement.
It features a TON of assassinations, and many of them are arguably more clever than the game's "core" assassination sequences (which, to me, was always one of the few ways AC2 fails to improve over its predecessor, and in most respects, actually disappoints). This helped diminish that sting a bit.
However, it and the Shipyard secret location (the only one of the three preorder bonuses I was missing) definitely did not merit the $10 I paid last night. I see that the DLC on PSN has been price adjusted to be more in line with the 360 version, so I think I'll email Sony about a $3 credit to my account for their mistake.
MS12, 13, and the secret locations should have just been packaged together for $10. It probably would have been more palatable that way.