The advantage of an indirect hot water tank are:
1) You only have to purchase/maintain a single burner
2) The boiler is used 12 months a year, reducing the problems associated with an inactive boiler for 6 months
3) As you noted, a single stack
I use an Weil-McClain 40 gallon (36 DHW, 6 for boiler circulation) indirect. I have 3 full baths in my house, me, my wife and three teenagers. We have never (ever) run out of hot water, even while running the dishwasher and clothes washer.