Guys this is very true, however to just reiterate the comments already made..
A linkage is used to Change the effective damping rate based on wheel position..
The same shock valving is now moving faster per unit travel as we get deeper in the stroke.. This makes for a stiffer and Slower shock the deeper into the stroke relative to damping...
It also makes the spring effectively stiffer... A linear rate spring behaves progressively...
Why? In simplest terms Motorcycles one need to support multiple weights..
-50% the weight of the bike, in level even throttle.... (We need some compliant)
-100% of the weight of the bike under the weight transfer of acceleration. (We need something firm and controlled to prevent hinging)
-<25% of the weight during braking.. (We need something light for when we hit a bump with no load)
So we need a system that changes its force and damping properties based on position to better suit each one of the suspension requirements based not just bump or jump absorption and subsequent energy release in very different conditions.. While there are many different aspects to consider, the linkage is fairly able to give reasonable compromise to each of these circumstances.. The main advantage of the linkage as it is extremely tunable.. A different motion and subsequent damping/force characteristic can be had at virtually any point in the stroke by tweaking different parts of the link and rocker..
When one considers how long the various manufactures have been working on this equation, its no wonder that the linkage less systems are so controversial.. The variables are equally complex, as we mix change is force relative to speed and position..
Finally.. Fork systems are not linear... They are progressive due to the air spring and pitch of the front fork in weight transfer..
BR,
Jer