Longer is better
I agree that longer rods can mend more line; in fact, that's rather self-evident, isn't it? And since line mending capability is one of the major advantages of spey rods, what's the point in minimizing this advantage with shorter spey rods?
But there's another, and I believe, more important advantage to longer spey rods: the ability to lift and place longer lengths of line as the initial movement of a spey cast. Recently, I've been paying more attention to this; I suspect that line lift and placement hasn't received the attention and analysis it deserves as a subset of spey casting skills. Early on, I found that there's a dramatic difference in the amount of line that, say, a 15-foot rod could bring back upstream compared to my 12.5' and 13.5' rods. (I should qualify this by adding that I prefer extended belly lines; probably the advantage is less when using, and stripping in, short-belly lines.)
About 109 years ago, George Kelson expressed it in one pithy sentence. Quoting from memory, he wrote something like this: "What line a spey rod can lift, it can cast."