Single or Twin Alternator setup ?
If Single then in the absence of any switching you would effectively connect the leisure bank
to the starter battery through this cable (and the terminal stud on the back of the alternator) -
resulting either in a flat starter battery having run the tv, lights, etc all night or some of the
starter current being drawn through the charging cable from the leisure bank.
With Twin alternators, one dedicated to each system, then there are good logical reasons
for doing as you suggest - any switch, junction etc. is a potential problem spot - high
resistance contacts etc., and this is indeed the practice commonly used in automotive
single alternator + single battery bank setup. However BSS requires that battery banks
can be isolated from their associated systems so AIUI you must have a switch somewhere
in the system between the battery and any attached equipment.
springy