Political advantage is debatable in this case. And that was his opinion even after the situation with Hermetia--you know it was Waymar who was most sympathetic, right? Viserys got her out of that mess, and had her back, because he gave his word, not even explicitly due to her situation but because his friend was basically asking for help.
I mean, his feelings on her have changed since they became friends, but still, really, it's a bit revisionist to paint him as someone who values desire over duty.