Ok… it’s perhaps a vendor-specific trick.
Or.. well, perhaps you fear that a MS in pure UL TBF is not going to communicate its IMSI to the PCU. For that purpose (in ALU), there is a way to force this sharing of IMSI, even if the MS is in pure UL TBF (it is called RA CAPABILITY UPDATE). I believe it is a 3GPP feature, because it occurs between PCU and SGSN, on a standardized interface.
Anyway, a pure UL TBF is rare (impossibly rare ?) 🙂