The claim that the "best" ending is achievable without multiplayer is false, unless BioWare's reps are engaging in some sort of sophistry about whether "synthesis" or "control-with-Shepard-surviving" is "best". At 50% readiness, one would need 8000 war assets to survive "control", and you can only get within a few hundred of that (successfully trick Wreav about the genophage cure, achieve geth-quarian peace being the most important decisions). This has been supported both by testing and by datamining.The multiplayer is apparently not compulsory, but the Defender achievement is multi-player only and the "secret" rubble scene is not unlocked by bringing back 7000 war assets at 50% readiness. Even the ending seems to be changed by the "non-compulsory" multi-player.
So the multiplayer isn't as segregated from the single-player experience as people might have hoped. It's still nice that you can get almost all of the achievements without playing multiplayer, unlike virtually every other game with a multiplayer mode on the market. Also, it's nice that the multiplayer is so damn fun that there's little point in avoiding it unless you have a bad/nonexistent internet connection. And I have a suspicion that the extra war assets will be made up with future DLC (similar to the way in which you get the Eden Prime war assets from the "From Ashes" DLC) if you really want to avoid multiplayer that much.