He did his duty to the Tokugawa clan by leading his troops (as the rear guard) (although he at least should have made the Takeda force retreat before abandoning the battle after the first battle at Takatenjin-jo Castle).
信長自身が出陣したことで徳川に対する義理（後詰）も果たしている（ただし、第一次高天神城の戦いで事実上見捨てており、少なくとも武田を退かせる必要はあった）。