It's not the FOSS contributions that are non-efforts. It's the things that were never released at all. What percentage of engineers at Microsoft work on code that has been or ever will be open source? How is releasing 10% of your code with no patent grant better than releasing 20% of your code with a conditional one?
Also, "no explicit patent grant has been sufficient" is no excuse for bashing a grant that has exactly that as its worst case. Any non-zero protection is still greater than zero. By way of analogy, would you yell at people on your team who you felt weren't working hard enough, then give a big pat on the back to one who played video games the whole time? Well, maybe you would, but an objective observer might think it seemed unfair. They might even wonder about what other motivations might be involved.