Yeah if Killzone 3 had a reach max level trophy, I would still be playing it. It's too bad they listened to the whiners and really watered the online trophies down.
I like the way Uncharted 2 handled the online trophies. Not required for platinum, but with the DLC they give you more trophies to keep playing the online long term. A lot of the trophies take a long time to get.
It's a good option because like others said, requiring online trophies for the platinum can break your ability to get platinum at a later time when the community is all but dead. Ex: Resistance 2.
If I was a developer I would only make the multiplayer trophies part of multiplayer DLC.