Palaven and Thessia are both worth thousands time more than Earth in the war effort. Since Earth isn't worth anything.
What determines worth, and to whom are they supposed to be the most valuable? If the Council races instead decided to never listen to Shepard, and hunkered down for the duration of the war and simply defend their puny planets, the reapers may take longer to wipe them out, but they'd wipe them out just the same. The only worth their worlds would have then would be the opportunities they'd provide the upcoming species of the next cycle to colonize, nothing more.
But I'm someone who hates the fact that Hackett/any human is apparently Supreme Commander of the allied war effort. I even seriously doubt humanity being given a council seat. It's just poor writing if you ask me.
Why wouldn't the humans be offered a seat on the Council? After all, it was the humans that stood between the Council and certain death, and the human fleet that stopped, or at least delayed, what would have been death for all of galactic civilization.
It's not that the Alliance fleets should have gone out of their way to help the other planets, but someone who is coordinating all the races should be willing to cut their loses.
In a sense, they kind of did cut their losses. They weren't sending anything to earth, and were simply hoping that the resistance that remained could hold out long enough for them to complete the weapon. They retreated through the relay, and put their effort toward building the Crucible. While their plan was to try to save earth, their plan was to basically try to destroy the reapers everywhere. Had that lousy Illusive Man not thrown so many monkey wrenches into the gears, there would have been no final assault over Earth, because they would have simply joined the Crucible with the Citadel and fired away.