From my talks with the folks at Horizon and my own experience I would have to say yes it's well worth the extra cost. I can get a pretty even temp reading across the entire smoker using the convection plate.
Ya know what; I think I did screw that up, I think you're right. I'll get some pics of the therms today.I'll get it right this time. I did end up replacing the Horizon therms though. I went to Tel Tru therms as I didn't find the Horizon therms to be accurate at all. That would be one thing that I would change, I can't recommend their therms.
I do use the sliding feature to dial things in. One other advantage is; you can run the firebox side hotter if you want. That's good if you're cooking two different types of meat that require different temps, say chicken and pork.