Well, two reasons really. First and foremost, since I had to modify the frame slightly to be able to use the 13 bar deep box, it was no longer a big enough space to hold the side burner. Second, while it seems like a cool thing at first, it takes away from counter space that I covet even with the left side table and two flip tables. Third, I find the side burners to be less than effective due to wind. If it is at all windy out, the side burners loose a lot of their heating capabilities.
I could further mod the frame to accommodate a side burner if I really wanted to. It wouldn't be that big of a deal.