Actually, they should pushing for a stalemate, Much like the Turians did at Pherios, they don't even have to keep all their fleets at Arcturus, Just enough to keep the Reapers away from the stations.
The Alliance in its entirety would not be able to keep the reapers from destroying Arcturus. They'd just overwhelm them and the loss would be catastrophic. The Illusive Man was right about one thing: If the reapers simply wanted to destroy everyone, they could do it. The one and only reason why it was possible to beat them at all with the Crucible was because they placed greater priority on harvesting than glassing planets. In any case, the Alliance simply does not have the turians' capacity to wage war in space. They simply don't have the ships. Basically, nothing matters without the Crucible.
Having the fleets amass at Arcturus, on top of having Shepard be stuck there for a hearing before a defense council would be kind of funny in how it spectacularly dooms the galaxy. Even if Shepard managed to get away (assuming the Normandy is there), the Crucible plans would probably be inaccessible at that point, because either Cerberus got the data, or the reapers destroyed the facility. The problem with this idea is that the plot involving the reapers themselves has to be rewritten to make this tactic work. The reapers would have to attack Earth first, while the fleets at Arcturus evacuate, rather than the other way around.