Hoping to clarify, you cannot make the chassis too rigid. That is like saying you do not need a good concrete foundation for a house. If the uni-body structure is not solid, you turn the entire car into an active part of the suspension. On top of that, you cannot control how active it is in the suspension system and this leads to compromised structural integrity which can lead to buckled panels, popped spot welds and worse. Think of it this way, these car were put together with spot welds. That is like making a pair of pants entirely of buttons and no stitching. Sure they will cover your arse, but stress them and the buttons are coming off. Adding chassis stiffing components is in the same vein as it allows the suspension to more accurately be tuned to the road at hand.
The roughness of ride that some complain of stems primarily from inadaquate shocks, spring rates and suspension travel. However, like everything in life, compromise and moderation is key to building a car that reacts solidly but does not beat you to death if its primary purpose is as a cruiser. If you are building a hard core handling vehicle, then large torsion and sway bar with big leaf springs are the norm. If you have a cruiser, then more moderate torsion bars with sway bars and a good gas shock will allow the car to have excellent road manners with slightly increased wheel rates and a fairly supple ride.
Don't forget that XV developed its stiffening kit through extensive lab and track testing. Their pieces are the result of a very analytic approach rather than simply putting pieces on the car and hoping they work. Their systems are designed to provide the least amount of compromise between weight, ride comfort, and handling. Go check out the changes here;
http://www.xvmotorsports.com/news/news_display1.cfm?id=37. IMO, they are worth it. Your putting 750 pounds worth of engine the nose of that car. I wouldn't worry about the extra 10-12 pounds those braces add.
