It could still be useful if the patents were part of a defence patent portfolio. (Ideally I'd abolish software patents altogether, or set a hard 10 year limit on them together with some kind of mechanism to curb the impact of add-on and submarine patents.)
Yes, it can be used defensively. If you breach the GPLv3 then in effect you also lose your patent license. The GPLv3 also forbids you making an opposing patent claim, so in effect it is defensive.
