Good points. I would give that a 3.5 is probably a more accurate measure with the arguments some of you have given.
I've been playing Wood Elves a lot in WotR and am used to pathetic Defense and mediocre Strength.
You just need to recognize your weakness and play to your strengths (hopefully you have something to make up for it!). In the case of Wood Elves it's LOTS of shooting (my forces is usually 100% bow armed plus packing Throwing Weapons) and I combine it with a few solid Epic Heroes and some Magic. I know when I get into a slugfest with Morannon or Uruks or similar that I'm probably going to get slaughtered if it's a "fair fight" so I avoid fair fights.
Rohan would have to do the same, relying on mobility and ranged fire as those are they're only real tools. This means careful play and that's always a fragile situation.
Another extensive bow-armed force ( Elves or Gray Company ) would do serious damage to a Rohan army, that's for sure. Regarding GE's comment "Rohan lacks too many important aspects of an army: artillery, uber-heroes, monsters, and magic." though, I'd say first that you don't need all of those obviously and some are questionable for any army. The lack of Artillery and Monsters are not two things I'm overly concerned with. We find Artillery doesn't always last long and is only moderately effective, and I put very little faith in any H2K models as they seem to last only one round of combat and don't really seem to pull their weight in points (Eagles being my only personal exception thanks to swoop attacks).
The lack of an Uber-Hero is debatable. Though they don't have an Aragorn or Gothmog, Eomer and Erkenbrand are each very good primary Epic Heroes and Theoden and Theodred are not too far behind...and all of those are much more affordable than the Heroes of many other armies so you could potentially field 3 such where you may otherwise only have 2, for example.
Magic is the biggest no-show in Rohan and would obviously require some outside help, but that's beyond the scope of this discussion. Magic is a bane of any army in WotR and can really wreck havoc. As you implied, the "subtle" magic of Tolkien's world has become the sambuca of WotR.
I still believe the mobility, with lots of ranged combat options (bows and throwing weapons) and good Epic Hero selection for the necessary Might can give you a strong combat force if played effectively. If you're facing a bow-heavy force you're going to have a major problem (thanks for pointing that one out
It's important to know your weakpoints). I notice though that there aren't that many bow-heavy forces due to the fact that bows are normally pretty weak against many armies in WotR. But there are some and most armies do have some ranged fire.
So a 3.5 on its own merits I would say. Some allies and you can quickly counter most of your weaknesses, but that goes for pretty much any army.
But just think of how COOL it will be to see four Formations, each 8 Companies large, of Riders of Rohan, lead by all the great leaders, banners waving, charging down on your enemy line.
Of course, they may not LAST long...but it will really look good while it does.