Even if the choice for a "happy" ending exists, the choice for a "dramatic" ending is not diminished. What makes one ending "optimal" and another ending "suboptimal" is entirely subjective, even in ME2--while one person might argue that the "No One Left Behind" ending is the "optimal" ending because everyone survived, another might just as strongly argue that their ending where Shepard's love interest died is the "optimal" ending because it really underscored the sacrifices Shepard is willing to make to stop the Reapers. It really comes down to player perference.
That's purely from a narrative point of view, and with respect to ME2 it mostly comes across as not as well done as I would have liked because I find obtaining the primary preconditions for it is rather trivial. That it's still a factor of the entire game's worth of play, however, is its saving grace. That is, it's not like the game says at the end "Okay, if you want you can kill some people off for no particularly good reason."
I
like that in my ME2 game, Thane died, and I think that that makes for a more interesting story. In spite of knowing this, I still consider the roleplaying "optimal" ending to be the one where everyone survives. I don't think it makes for a more interesting story, but it's clear that my Shepard would have preferred everyone to survive. If he had the clairvoyance to know which decisions would result in the death of various characters, he would have made different choices.
With respect to ME3, a game where the choices are at the end themselves (rather than a culmination of the entire game with no proactive choice at the end), the idea that had I done everything and I am presented with a choice to say "eff off" and it works compared to the other choices defeats any of the interesting aspects of the choice. At best they become "that was interesting from a narrative perspective" and less of a "I find it interesting that in spite of all the challenges, there's still a great cost that must be paid in other to win this thing."
This is compounded if there's explicit closure. Without explicit closure, the player is arguably put more into the head of Shepard in that you have to make your decision and live with it, without
ever getting any extra metaknowledge to influence your decision. The focus shifts to the choice itself, rather than the consequences of the choice. I think people that liked the ending to ME3 are the ones that were able to enjoy that focus, whereas there's definitely a group of people that did prefer to have more closure and they are the ones that do not like the ending (at least the original ending).