I guess the question I need to understand is 'how valid is voltage as a control input'?
The Ironman is 13.7V, NL and IBS are 13.2V IIRC. If, as the Portal link says, a healthy alternator can quickly buoy the starting voltage above the 13.2 or 13.7 threshold AND the hotel battery is significantly discharged then the circuit could be prompted to open too soon before the starting batt is fully charged. In my case, the 31 as a hotel may take a good amount of suck and there could be scenarios where it leaves the starting battery incompletely charged.
There's also good discussion about the 12.2V Ironman cutoff as being too low, partially depleting the starting batt before clamping off. On the one hand, it could be a nice 'feature' to have both 31 and 34 powering hotel loads until the common voltage drops below 31. On the other, reliability trumps in my book and a depleted starting batt defeats the whole purpose of a dual system. Then again, both batts are marine deep cycle in my system so maybe this isn't that big of an issue.
Further, in typical deployments, it would be rare to have a short drive cycle unless it's just a re-charge idle-only cycle.