hi jason,
unfortunately, your answer is probably not correct : all security checks are perform by the core network only (HLR/EIC)
The BSC has one reason to need the IMSI : it’s to be able to perform the Paging Coordination.
There might be other reason, but i cant find any. So I suspect that this sending of the IMSI is rather recent in networks (let’s say 4 or 5 yrs)
Regards,
pix