redundancy of second stages is not necessary except in ice diving. With the exception of freezing, first stages are far more prone to failure than second stages. The issue with the combo inflators is that they are both ****ty inflators and ****ty regulators. For Ken's application on a rebreather, he will essentially never orally inflate his bc because it requires coming off of the loop, but for open circuit diving, the last thing you want is to be trying to maintain buoyancy, while breathing off of your inflator, and then have to make an ascent using the Air2. They're also big, bulky, and require special hose connections *which on a CCR you will already have due to the high flow connections for ADV, BOV, etc, but will not have on normal OC regulator setups.