If Yahoo has any "brand value", it's with users who are very non-technical, and have no idea or opinions about patent lawsuits.
So brand isn't a good argument against this suit for Yahoo.
Morality might be. But it's hard to get people to act against their financial best interest over the long term, so I still feel the real issue is the patent system.
besides, I would argue that the cost in both goodwill and brand value of pursuing a strategy of patent licensing far outweigh the potential benefits.
[1] http://en.wikipedia.org/wiki/Business_judgment_rule#Standard...