I think the argument in favor of patents is similar to that used to defend any capitalistic endevor
That is that someone with genuine ability deserves to be rewarded for exercising that ability. Ultimately, if no one is rewarded for their ideas, people will not work so hard to come up with them.
It is sad and true that there are folks out there who will try to use the system (and the courts) to their benefit, without any particular creativity on their part. That's life in the big city.