How can the Tower do what Rand's vastly superior forces won't be able to do anyway? Rand HAS thousands of channelers. He is allied to the Sea Folk, he has 11 clans full of Wise Ones which is about 3 or 4 Tower's worth even after accounting for attrition, and probably closer to five times the Tower's weight (Recall the reactions of the sisters accompanying Perrin to the Wise Ones' enumeration of the Shaido channelers - "Half the Tower from one clan." There were no Wise One equivalents of the Meradin, so the other clans probably have similar numbers). Not only that, they are not constrained by the Three Oaths, and understand warfare and combat better. We have seen that Aes Sedai are not all that good at fighting, so they cannot bring much to Rand's aid. How can the 1,000 channelers in the Tower be such a significant weight next to an equal number of Asha'man, 5-6,000 Wise One channelers, plus the double weight of the Kin. Don't forget who has the leaders of that group, and thus the means of rounding up their numbers - Elayne. Rand hardly needs Egwene to sway Elayne in his favor.

All in all, the Tower united benefits the Tower. The additional help they can bring to Rand is negligible and so laden down with caveats and strings attached that a good question can be made as to just how useful it will really be. The re-unification can only be seen as a good and necessary thing from the perspective of the Tower, since it enables them to ride out the world crisis to their best advantage.