Reg, Rigid does make a set that attaches the springs to the axles. It is the set with the red Links, Izzy has it on his daycab. I have owned this set along with ScaleArt and Oswald sets, none of mine have hooked the spring packs to the axles. I agree, they should be but I think that limits the axle travel. I also think all of these kits should have a brace or cross member running under the suspension tying the two bolsters together, haven't seen that done either.
My ScaleArt sets on my Wedico articulate very well.