One option is to point out that open source has its OWN set of high-powered attorneys -- check out OSI. IBM is known to defend open source because they would also have to pay.
Simply put, the more and larger companies using a piece of open source, the less likely someone will attack it -- because more and bigger companies means more money and larger legal teams to defend and countersue.
It's also important to note to them that the bulk of the patent suits result from someone using a product (that in most cases was not open source to begin with it) as a basis for a proprietery, for-profit system.
Due to the nature of open source, few products fall into this area. There's no
profit to be made in suing someone with no assets.