Even if they send their fleets, I'm not sure it would do much good anyway. Generally, I pin the defeat at Wolf 359 on the fact that the Federation couldn't hustle enough reinforcements to the area fast enough, and Wolf 359 had to be fought with whatever ships they had on hand. After all, in DS9 we heard about massive ship losses in the 500+ range, yet they shruged it off. If the Borg are coming in fast enough, many of those ships would't get to the battle in time.