The issue is the ambiguity of "defensive" action gives the assignee of the patent relatively broad license to act without the consent of the inventor - and the inventor can be suborned later, too. (Trust me, that happens.)
Do you really believe that will happen? When investors are asking why some pinhead inventor is holding up their profits? You can't force everyone involved in a company to think this is a good idea, and therefore, it isn't.
Edit: (For some added color, imagine Kevin O'leary of Shark Tank fame sitting on the board of a company with some of these patents).
The article we're commenting on is an investor excited that software patents will stop holding up profits.
I expect investors to be rational - and to seek profits where they are available given the information that is currently on the table. In some cases, that means they will say, "you know what? Screw that doc - we're selling these puppies."
In certain situations, I could see investors trying to undo this agreement so they could unlock that value. If the value is great enough, they will try really hard (and given how vague this agreement is, I bet they'll succeed). That's all I'm saying.
In the end though, I don't see how this agreement is anything more than twitter saying "we won't sue people over patents, and we won't sell them to people who will". I don't see that as particularly significant. Just that twitter isn't pursuing licensing revenue as a strategy.