Okay. Let's take a really straightforward example. Lets say that you have a sheet metal main gear. You look at it and decide that you don't like how springy it is. You decide to make it "stronger" by sticking a rod between the left and right gears.
That WILL stiffen the gear, however, you've likely reduced the load bearing ability of the gear by concentrating the stress that WAS spread out over, say, a foot (the gear flexed), into a space of a couple of inches (the rod). On the first hard landing, the gear would most likely fail by ripping out the rod.
In the case of the rudder, vertical stiffeners between the regular stiffeners could concentrate stress at the ends of the vertical stiffeners. i.e. under load you might cause creases in the skins. I don't know. I didn't design the rudder and have no idea what loads it sees in flight. It seems like it'd be OK, but a phone call to vans would yield a definitive answer.
The other problem, of course, is anytime you change the weight and stiffeness of a control surface, you become a test pilot and need to worry about flutter.
-john
