As hellosailor and W3GAC have reiterated, voltage spikes from the starter circuit can and do damage sensitive electronics.
But, I guess no matter who says it....again and again...some folks just won't believe it or will draw from their own experience in getting away with it.
So, my last ditch try: IT'S A VERY BAD IDEA TO COMBINE THE HOUSE AND START CIRCUITS DUE TO POTENTIAL DAMAGE TO SENSITIVE ELECTRONICS.
Yes, there are "safeguards" ... capacitors and spike spikers and make-before-break battery
switches and all the rest.
HOWEVER, why even take a chance when there's absolutely no need to?
Keep the house and start circuits separate. Use an ACR or voltage-follower device like an EchoCharge or DuoCharge to automatically maintain the start battery.
With these devices, operation is TOTALLY AUTOMATIC. NO BATTERY SWITCHING IS NECESSARY. Here's one way to do that.