Vol'jin's words brought everyone to a realization. Shattrath was certainly a force they could put to good use. The Naaru? The Horde did not trust the Naaru. Shattrath's existence was merely a pretext for the Horde. As long as Quel'Thalas's main force was not wiped out and Kael'thas died in Outland, the Orcs would have an opportunity to exploit the relationship between the Forsaken and Quel'Thalas to draw the Kingdom of Quel'Thalas into the Horde.
Because of Dalaran, Quel'Thalas could not be on good terms with the Alliance again. All of Silvermoon City-or rather, Eversong Woods-was also hemmed in by the Plaguelands at the northernmost end of the Eastern Kingdoms. Quel'Thalas joining the Horde to protect itself after losing all its elite troops was also part of Thrall's calculations.
This was the best outcome for the Horde. After losing their main force and being drawn into the Horde by Sylvanas, the Blood Elves would be neither particularly strong nor particularly weak. They would not be so powerful that they upset the Horde's internal balance, nor so weak that they were useless. In the end, the Orcs would still be in charge within the Horde.
Moreover, because the Orcs were fully and directly involved in the operation to eliminate Kael'thas, Quel'Thalas would have no legitimate grounds for retaliating against the Horde. Once Quel'Thalas joined the Horde, its strained relationship with the Alliance would allow Thrall to use the name of the New Horde to wipe away the Orcs' invasion of Quel'Thalas during the Second War. He could also create an opponent for the Alliance, rapidly strengthen the Horde, balance the two sides, and once again let the Orcs, Trolls, and Tauren rest and recuperate.
This was what Thrall truly had in mind.
Allies? Politics never concerned itself with such things.