If you read the shock theory stuff it's accurate that the shock has it's greatest effect in a true vertical installation. There are charts that show the reduction in effect in relation to increasing angle of mounting.

That's all well and good. Then the reality of necessary compromise sets in. On a tubbed car, whose original intent was drag racing, you don't get much if any body roll (hopefully), and the pavement is relatively flat (again, hopefully), or at least not enough to be a significant problem. On the road you have to deal with turning (resultant roll) and uneven surfaces (potholes, shoulders, swales, crown, etc.). Axles (either solid rear, independent, or front beam axles or control arms) swing in an arc, and in the case of a "solid" rear axle, on say uneven pavement, think of one side as "fixed" (the none bump side), and the other being the outer end of the arc. That's why, if you look at a professionally engineered passenger car for instance, the shocks are angled. Sure, it's less efficient for straight up and down, but it's a reasonable compromise for the wide variety of driving conditions encountered. The shorter the "axle" the more vertical the shock because there's less arc effect.