I'm not an expert on the matter, but I thought that the boundary conditions of the Arduino, mainly limited CPU power and limited memory, make it less suitable for very defensive coding and more suitable for (mild) programming by contract.
While it certainly does not meet the requirements of keeping an Joint Strike Fighter in the air and other life-and-death critical applications, think it is usable in may other commercial environments. Besides, there is the enterprise code a programmer would like to write, and there is the code he writes under time constraints, changing requirements etc. As said, compared to some commercial code, the Arduino libraries aren't always that bad 
Of course, this is all pretty off-topic...