You probably did not see all the issues that come up with zones and portals. Let us take shadows. Some shadows have to go through portals, some others dont have to. Some lights might be switched on even if they are not visible on their own but they create shadows and lighting behind a portal. There is geometry scissoring and maybe background streaming.

It is not a trivial task if you do it right.

But it gives more freedom than compiling BSP maps for hours or days.