Greer is a 4th line energy player who doesn't hurt you and can put up the occasional goal. He is not a tough guy, or a good fighter.
Lomberg is a fighter, and more of a rat. He is decent defensively and knows when to pick his spots so he doesn't hurt the team, but he is more exuberant and more of a presence. Greer experessed interest in staying, but the Flames didn't re-sign him. I find it hard to disagree - he doesn't really fit a spot. On a contendting team with a hole for a solid energy guy that doesn't hurt you? He fits. On a team with a surprising amount of depth without much real toughness and requiring more leadership? Lomberg is a better fit.
Pospisil can really use a guy like Lomberg who can help teach him to pick his spots a bit better, and maybe protect the kid too. Klapka might be in the lineup too. I can only imagine the fits a line of Lomberg - Pospisil - Klapka would create for opposing teams.
Rebuilding teams need character guys. They are underrated. I am sure that Florida would have loved to re-sign Lomberg back because guys like Lomberg help keep a contender loose, but contending teams can't match those contracts and need to recylcle through the bargain bin on the 4th line players. Not to say that Greer is bad - he is good in his own right, just cheaper right now.