Except McCarthy fore-checks about as much as Havlat does. There are plenty of guys they can get to center the 4th line. The natural center stuff is the weakest argument for keeping a guy in especially when McCarthy wasn't even playing center in San Jose until midway through this season. His penalty killing is replaceable. They had gotten by without it for a long while before he was called up.
His skating is not what it was before but I don't see McCarthy's skating as being much better. Brown is much faster than either but is horribly ineffective. And that's an issue with McCarthy. Even if he is a better skater than Havlat, he still isn't as effective as Havlat. And Havlat has a long history of very good defensive play in his career so to say he's hardly a difference maker is not giving enough credit where it's due. Even in a season like he's having now, he still manages to do well defensively and not give goals up. McCarthy cannot say that...he's not good defensively.
Your comparing a winger to a center defensively. Apples and oranges.
Havlat may have been something in his career, but here and now he is a worse choice than McCarthy.
McCarthy does forecheck. That line is but to forecheck. He's not first in, but that's that lines identity. Havlat would come in and play completely different from those guys. It won't work.
Notice I didn't say his speed made him effective. Just that he's a better skater, more balance better edges.
In terms of possession stats he is equal to brown with around the same amount of minutes.
Instead of figuring out why McCarthy fits better with how this team plays (which is fairly obvious) tell me how what you've seen from havlat will really make a difference in the long run in this current bottom six.