You may be misreading the procedure. Ours says there have to be four conditions met before shutting down engine #2 after landing. These 4 being:
1. After landing check complete;
2. Cooldown period complied with (if necessary);
3. APU AVAIL light is on (because our birds don't have that much-discussed mod);
4. Aircraft moving in a straight line.
Once all those are met, THEN you go into the Engine 2 shutdown flow, which is Y pump on, engine A/I off, master off.
The reason we were given for #4 is that during power transfer it is possible for the BSCU to momentarily lose power, thus rendering you brakeless (again, momentarily) which would be a bad thing in a turn. Whether that's a legitimate concern I know not.