Now, do we prefer to play with a ready-made software that will always hide how it's made, or work with a free software that shows us everything? The choice is simple enough...
Have you ever had a look at the PD internals? You will easily find that everything is open but nothing is documented and as long you haven't spent months on trying to understand what's going on, you won't notice much of a difference between PD and Max in this respect. If one is willing to dig into it, one can have great benefits, but my experience tells me that it's not what most people using real-time-systems are interested about. What concerns programming externals, documentation is available for both systems. If you compared just the quality of overall documentation the choice would be simple enough.
greetings, Thomas