That sounds like more of an issue with the war asset system than the plot itself. The fact that multiplayer allows you to just heap on points through promotions is, I think, the real problem. But then, the same is true of the DLC's, which can net you hundreds of extra points themselves. I guess this is the problem with putting any kind of real details into the Crucible while also having to build the collective strength of your allies. Like, if you gather the most assets in your fighting force that the game allows, you could still get the worst ending, because you didn't gather enough wingnuts to hold the Crucible's reaper discombobulator together.
Never cared much for the War Asset system, despite my many, many issues with ME3 it's this system that I feel robs ME3 of most of its fun. If your'e a gamer like me who has mild OCD and meta gaming tendencies then it becomes impossible to play ME3 and enjoy it at face value for the things you are doing ( combat, interacting with people, exploration ) because the only thing you can see is that stupid bar ( war asset metre ) and the numbers associated with it. I don't see npc's on the Citadel, I only see war assets, 10 points here, 5 there, 20 over there. I don't see planet exploration or even planets for that matter ( I don't even stop to read the planet descriptions any more, something I loved doing in ME and ME2 because I'm too rushed by the Reaper evasion mini game, it's just get in and get out before I get caught ) I just see useless junk I need to fetch in order to get another a few war assets. Side quests just become numbers and the main games storyline at the end of the day just comes down to more numbers, everything about ME3 comes down to that stupid bar. What Bioware have done with ME and ME2 is that they had pulled off the greatest magic trick in the entire world and impressed everyone with it but in ME3 they took us behind the scenes, showed us how the trick was performed and now there is no longer any wow factor, the magic is gone and it all seems very ordinary and mundane now.
ME3 should have never been about numbers but that is what it is. Make this decision and you get 20 war assets but make the other one and you get 50. It puts a certain weight on decisions that should not be there. If you have OCD and meta game like me then the message that Bioware is sending you is that Y decision is more correct than X decision because you get more points for it. In my head I interpret this as Y decision is cannon and X is not which is something that I don't think Bioware ever intended for gamers to think. A different decision should lead you down a different path but it should not result in a better ( or worse ) score than the other choice you were offered.
I have always maintained that ME3 has less replay value than it's predecessors and it is because we now now that the only thing that separates all your previous choices in the trilogy from the the choices you did not make is a number. There are very few true branching paths in the ME trilogy. At the end of the everything comes down to a scoreboard.