Here is a far fetched scenario where an ACR type relay could be dangerous.
#1 You accidentally deplete your house bank by leaving the fridge on over night after being on teh hook for the last three days.
#2 You switch to the start battery and fire up the engine. Good thing for start/emergency batteries.
#3 Once the alternator ramps up the ACR relay combines both banks and the start battery, being nearly fully charged, now equalizes voltage, or attempts to, between the very large house bank and the start battery.
#4 You motor out of the anchorage and realize you're not spitting any water out of the engine exhaust so you drop hook, shut down the motor and clean the sea strainer.
#5 You finish cleaning the sea strainer and go to start the motor but all you get is click, click. This is because your group 24 battery with roughly 70 amp hours just tried to charge a 450 amp hour bank of dead batteries and now the entire system is at about a 10% charge, mostly dead and unable to supply the 160+ cranking amps needed to start the motor. The short run time and the resulting alt output from this short run time was not enough to overcome the dead house bank enough in the five minutes you ran the motor..
Again, far fetched but this could happen and Murphy is always waiting...
With an Echo charger this would not happen.
Yes, but in all fairness and in the spirit of full disclosure, the above scenario can be eliminated by simply removing the battery switch such that you have dedicated, separate start and house banks. In point of fact, the switch is completely unnecessary and only serves to create problems which is what i think the writer of the above is trying to say.