"If Assignee needs to assert any of the Patent claims against any entity for other than a Defensive Purpose, Assignees must obtain prior written permission from all of the Inventors without additional consideration or threat."
At which point whoever twitter was suing could claim that they had violated their own agreement so they don't have grounds to sue.
The agreement would likely be private, so as long as both parties keep their mouth shut about it (which they're incentivized to do), there is little risk for them.
Personally, I see this as a great potential bonus source of revenue for the inventor down the road, selling the right to use their patent offensively.
Most companies won't do it though, since it restricts the potential market for their patent IP, which decreases the value of their assets on their balance sheet.
The common law principle of privity of contract (see http://en.wikipedia.org/wiki/Privity_of_contract) means that contracts can only be enforced by the parties to that contract (although some jurisdictions have changed this with legislation). A defendant in a lawsuit against Twitter cannot assert an agreement between a third party and Twitter against Twitter.
Well, if this sort of agreement becomes widespread, it would effectively opt them in to being sued by other companies who have patents assigned under a similar contract. Also, it would be in breach of any other IPA contracts they've made with other inventors.
It's a neat concept. The more widespread these contracts become, the more litigation you expose yourself to for using your patents offensively.
No, it explains how the purchaser of the patent can get the inventor to go along with their offensive patent lawsuit, which is necessary BECAUSE the inventor maintains veto power. If that weren't the case, the inventor would have no say in the matter.
But what's the point of the "no consideration" bit? What's the point of two parties agreeing that they won't make a new deal later, when they can anyway?
They just have to say if pressed that there was no threat or other consideration, and that they agreed to let the other party use their patent offensively. Then collect their totally unrelated consulting fee.
"If Assignee needs to assert any of the Patent claims against any entity for other than a Defensive Purpose, Assignees must obtain prior written permission from all of the Inventors without additional consideration or threat."
At which point whoever twitter was suing could claim that they had violated their own agreement so they don't have grounds to sue.