Ok, so lets assume for a second that IBM really wants to benefit the OSS movement by copylefting most of their Software Patents

(the fact that i believe that SW Patents are BS in the first place is another issue).

They file it, and maybe get it granted rather quickly.
In the US its 3.5 Years until they have to pay an annuity fee for another 3.5 years.
lets just assume, instead of paying this, they drop it.
the Patent is still on record as prior art.
Meaning that any Patent Troll, trying to re-patent this "invention" can be easily blown out of the water.
So: Mission accomplished - Software out in the open, no chance for Patent troll to cash in on it.