Silence,
Yes, to reduce the signalling load, and more precisely : the paging load. Indeed, the CS paging and the PS paging are both sent on the same resource (the PCH). This is the only radio channel which is (directly) affected by the LAC and RAC dimensioning.
Now, in reality, the PS Paging load is so low that it is useless to distinguish the RA from the LA. This holds true as long as there is no PCH congestion due to high PS Paging load.
So basically, simple settings are : RA = LA … and check your PCH load.
Cheers,
pix