There is one thing that I think is strange about this rule or interpretation. How many bowlers across the country throw the ball thumbless for strikes and with the thumb in the ball for spares. If it were me that is what I would do. So I guess that is not legal. If what is discussed here is the USBC interpretation, then it is time to make changes to the rule.
Something that may create some bowlers wanting to do this two handed delivery might be the recent international TV event. In the final frame with the game lost, Chris Barnes threw the final shot two handed and struck. We all know from the discussions that this is legal in the PBA and apparently in international competition. Just not in our leagues. Chris was bowling the master of the two handed delivery and just got spanked, so he did it as a complement to Belmonte, not to be funny.