Astarion has danced with people who stepped on his feet, who've never danced before, who were drunk to the point of near-unconsciousness, who just used 'dance' as a pretext to feel him up - and any combination of the above. Jacob's slightly lesser aptitude is, in that context, absolutely harmless. He keeps him close as they move, casting the occasional glance and smile to Rosita as he turns to face her before sliding away.