Yes I built lift kits for atvs and know how compressing the spring to get a slight increase in lift is achieved.
If you are getting 2.8" of lift by compressing the spring the truck would ride like crap not better it would be way stiff.
The bils are just lifting the bottom instead of the top as the normal spacer is doing, if I am wrong I will just go put a spacer under my spring and get a 2.8" lift and so would everyone else.
Now going from a 2.1" lift to a 2.8" lift might just be compressing the spring but that first 2.1" is coming from a longer strut.
Not saying the bils are not a better way to go because you get a better quality strut then the oem one but if you think your ball joints and other parts are going to last longer your wrong, its still increasing the angle on the a arms which is putting stress on all the parts just like the spacer does at the top.