well, proposed measure is firstly imagined to deal with realm hierarchy and claims in general, so that everyone takes rebellion very seriously.
banning is firstly consequence of direct breaking of formal laws, while rebellion presents very special circumstances.
there are also many technical issues of banning - that is at discretion of judge, and finally, of course, issue of changing allegiances.
who would even try for rebellion if knowing that most of lords will simply leave the realm?
idea is that rebellion presents real danger for all, so no-one should be able to avoid its consequences.
if you support current leadership, help them to prevent rebellion before it starts. if it starts nevertheless, fight for your side by all means or you will lose everything.
this could also create interesting conflicts in realms where much of ordinary nobility is annoyed with fat and complacent lords.
