Flexibility and the rod collection
To me two key issues are 1) flexibility of use for the different presentations you use, and 2) how many rods you will have in your arsenal. It's possible to take this question too far and try to define a "do-it-all", but I question the value of that. Here's my take:
1) Consider the different presentations you fish--sinktips, floating line w/ long leader, indicator, chuck-n-duck, underhand casting with heads, overhand casting with heads, etc. I suggest that you aim your choice toward your primary use, as some rods handle some uses better than others. I like a switch rod in the 11' (6-7) range for indicator nymphing when trout are in the picture and the casting is no more than 55'. It's light, handy and doesn't feel like overkill on a 12" pre-migrant or lake-run brown. Stack mending a two hander is a breeze.
I prefer the longer rod for all of the other uses, with my two primary long rods 12.5' 7/8 and 14' 9wt. I disagree with Dan above about the long rod being appropriate only on big water. Before looking at the presentation details, consider that the fish-catching "lethality" in the Great Lakes isn't so much about distance casting as it is about line control. Sure, I can boom sinktips across the Muskegon, but the point is that I can control them better with my 14/9 than with the shorter rod. And in smaller rivers, that 14' rod gives me superior line control too. I will often fish smaller rods with smaller flies, but the big rod rules for line control, tips, wind, and bigger flies. I can mend and fish a two nymph rig 10-12' down under an indicator, 75' out with a 14' rod, and there are a few places where that will get fish when nothing else will. But I also like to fish tips, and can do the same thing. On small water the longer rod lets me avoid having to strip and feed line so much, and it lets me thread the needle with presentation. On a 40' river in January, with a longer rod, I can dance streamers in the faces of steelhead, and cover water laterally, while standing in one place. I can't do that as well with a switch rod. If I'm fishing indicators all day, the switch rod gets the nod. But if I'm going to fish other presentations, or switch around, the long rod wins. The only situation where I'd look to a medium length rod as key is overhead casting of heads. I'd love to cast the new CND Atlantis for browns and steelies in our Lake Michigan surf.
2) About the arsenal: If I had to recommend one rod for GL, I'd be right with Juro on a 13' 8/9. Long enough to give the line control advantages. Comfortable length and weight to fish Skagit style, adaptable to indicators, not unwieldy on small streams, and capable of any of the above presentations. I think the biggest mistake made by new GL speyers is thinking that a 9wt spey is only for salmon as they view 9wt singles. A 9wt spey rod can be a ball with modest sized fish too. But 8/9 is about entry level weight for fishing decent sized sinktips and/or dealing with much wind. However you may find that having just one spey rod is like stopping fishing after catching just one fish...kinda hard to stop.
3) I've split the difference in my "arsenal". My 12.5' 7/8 is great for what the westerners would call summer conditions--low water, floating line, spooky fish and smaller flies. It's also great with carp and river smallies, and fishes indicators effectively further than the switch rod and with less mending. When the fall and winter winds blow, or I need to fish tips deep and/or bigger streamers, the 14' 9wt is the tool--and yes I'd take it any day over the 13' 8. But it's probably appropriate to mention that I'm generally a long belly speyer. The heads are fun to cast, and stripping produces well, but fishing the long belly avoids guide icing--and I often fish when the river is making ice, not just icing guides.