They all have it in a way, through the air cycle machine. But that's not the same as something like a Keith freon system, with a couple of blowers. Believe me, it makes a big difference.
So, having it now and not having it in the past, I much prefer to have it. Especially in the midwest during the summer... That said, I don't know how much it costs to put it in, and I suppose I might not pay to add it. However, in my opinion it's definitely a plus.
Incidentally, the Jetprops with the "Enviro" system all have basically the Keith system with no air cycle machine.