In wa-lā yughnī ghanāʾī wa-mashhadī, the poet does not regard mere attendance as enough. Ghanāʾī means helpful service or effectiveness, whereas mashhadī evokes his presence at consequential occasions.
One person may appear but contribute little; another may both appear and act usefully. Ṭarafah claims the latter status for himself. Reading ghanāʾī as singing would ignore the context of public service and ability.