I can explain that in the manual. Long story short, when you drop to 4:11 you lose all compatibility with the mechanics of a locking center differential.
Why must the viscus center be used with the 4.11 gearing? I've found that the viscus diff in my legacy is completely pathetic and wouldn't want it offroad.
For me, the huge gearing drop was well worth it. The drop to 4:11 combined with better hi/lo combined with legacy 2.2 is really, really nice for doing jeep trails.