Here is the problem:
We want to have a fluid and fast game, so we play smaller guys. Playing smaller guys only encourages the other team to do isolation post ups. Isolation post ups and the fouls that ensue slow down the game.
Thus we have accomplished little outside of giving the other team a free throw and rebounding edge.
Marion's ability to defend the 4 is an aberration for players his size, not the standard. You can't just simply plug an even smaller player into that position and assume that they will succeed.
It's certainly a mixed bag. There is little doubt that teams with well developed low post offenses can exploit the Suns small defenders. But unless their low post guy is a good passer, they risk lots of turnovers when dealing with the double teams.
The problem is greater for teams without a well developed low post offense. Many teams find that their entire offense grinds to a halt when they spend 24 seconds trying to make an entry pass rather than taking open shots.
In many cases, it is like a game of "chicken". The Suns struggle to stop their opponent's big guys and the other team has cannot stop the Suns using bigs. If the Suns shoot badly, they get crushed, but few bigs can stop the Suns if they shoot well becuase they cannot defense five guys when four of them are recent all stars.
BUT, this just doesn't work against the Spurs or at least hasn't. The Spurs do a better job of running their inside-outside/drive the basket offense than anybody in the NBA. That's why they have two rings in the last three years.