One possibly self-interested effect could be that it may make engineers more willing to go along with applying for patents. A lot of engineers feel very squeamish about software patents, and try to avoid applying for them or letting themselves get coerced into applying for them. A commitment by Twitter that they won't use them offensively w/o the engineer's permission may make recalcitrant engineers more likely to submit/sign the relevant documents.
I said specifically I'm not intending to undercut their motives. Even if that's true, I think it's worth understanding how it impacts then strategically.
