To my knowledge all standard 737 A/P installations are at most CAT IIIa fail passive. Options for CAT IIIb exist but are rare. But I dont understand the point you are raising, yes, if you just use VOR/LOC and V/S (or VNAV!) you will avoid the bus split - the QRH even tells you not to use the AFDS approach mode in the TR UNIT checklist. The point they are making in the OM is that if you have a catastrophic electrical problem during an approach you retain one half of your DC system (which after all encompasses all critical instruments) instead of frying it all. Does that make any sense?