[Chris P.]

Actually, upon a second consideration, there is an advantage to horizontal mounting which Chris Liokos has mentioned already. To give a specific example, braking. There are longitudinal loads (in the x direction as set forth by capaci) which will be held better by horizontal mounting. Since on the upper arms in that photo there are no sway bars, springs, or shocks, the suspension arm forces are purely in plane with the suspension arm (depending upon the slight angle of the outer ball joint if there is any, and the design of the ball joint). However, large enough inner rod ends in a vertical position will hold these forces that tend to "pop" the ball out of the socket just fine. These maximum loads are always provided by rod end manufacturers. Look at the outer rod ends on the lower front arms of just about any race car. They are in the "horizontal" position for steering movement and carry vertical loads from springs / dampers / shocks. Also, the bottom arm takes higher compression / tension loads then the top arm due to simple geometry reasons.

One reason for using a vertical position is simply to have enough room for travel. If you have a very short suspension arm and appreciable wheel travel, you might require a vertical orientation unless you use a more expensive high misalignment rod end, if it will even fit.