So you give RPX protection money and they leave you alone. This is what Intellectual Ventures does. Why is it ok for RPX? (never heard of RPX before btw)
Or have I missed something obvious?
See also: Twitter Patent Agreement http://news.ycombinator.com/item?id=3853949
There's also value add whenever you have centralized expertise: think PGExperts, Percona, and EnterpriseDB (mysql and postgres consultants) - if I'm running a company whose core business is not in the data, it's usually more economical to outsource that expertise than hire a 'superstar' DBA. Same with RPX: if your core business isn't IP law you might as well outsource that.
This is why consultancies of all shapes and types exist in the world, after all.
Companies spend millions just to defend a suit whether they win or lose - that's why they settle. The genius of RPX is that they basically identified that as an inefficiency aka market opportunity.
A lot of startups could learn from this model: taking an existing market and making it more efficient. That's usually a better bet than building an Instagram or Twitter analytics company - the big caveat of course is you actually have to have domain expertise in an industry to do this type of startup ;)