One of the problems with the factory supplied dual rate sliders on coilovers is that they are plastic. This is good in terms of protecting the shock body, but bad in that they can flex or break.
When the springs push against them they deflect causing the springs to bow. This can be mitigated somewhat by aligning the two springs so the deflection is minimal, but they don't always stay that way, and when compressed tend to bow anyway.
The result of this bowing and deflection is excessive wear on the shock body, or even shock failure if it gets bad.
There are aluminum dual rate sliders available. These get rid of the deflection and bowing issues, but then you have wear issues with the aluminum on the shock body.
The All German Motorsports (AGM) Dual Rate Slider
offers the best of both worlds, a strong two piece 6061 body to prevent deflection and bowing, along with a precision fit replaceable insert.
This dual rate slider has a long body ensuring it cannot twist against the shock body, helping reduce spring bowing even more One real nice feature is the upper and lower sections rotate, so as the springs expand and collapse they are not putting rotational force on the slider. This also makes adjusting the coilovers easier.
The details have been well thought out on these sliders, even small things like the insert falls short of the upper part of the slider, which is thicker, this allows the slider to contact the stop ring without any damage to the insert.
There is a price to all this awesomeness at $400 for four sliders, but if they extend the life of your shock bodies, or save one from completely failing, then it maybe a price worth paying.
Damage caused by spring bowing to the shock body
AGM slider compared to stock slider
The two sections allow for easy replacement of the insert and reduces rotational forces on the slider
Installed on a King coilover, they are also available for Fox and Bilstein coilovers