Removing the midvalve shim stack and replacing it with a check plate is back-dating your forks 10 years !!
The '92 honda CR line was the first to use a midvalve shim stack, it didnt work too well, but that is where it all started.
The midvalve shim stack is one more area for suspension tuning, when the midvalve is set up correctly (either for woods or MX) it makes for a much better handling motorcycle.
Some suspension companies choose to remove the midvalve rather than test and refine, its a lot easier just to use an old set up, than it is to take the time and effort for getting the midvalve working properly. There is alot of suspension performance potential in the midvalve.
The DRZ400 comes with a check plate instead of a midvalve shim stack, owners usually complain of soft mushy forks, when a proper midvalve shim stack is installed it makes a huge difference.
Take care, John