The desire to reason about the precise behavior of a program and the desire to take advantage of different behavior on different platforms are fundamentally at odds. Like I said, there's a broad range of just how much of the specification you leave up to the implementation; it's an engineering trade-off like any other.
My point is that by leaving a behavior to be defined by the implementation you're not making it any easier to write portable code, but you may be making some optimizations impossible.
That's not entirely true. Regarding portability, the layout of structs for example is implementation-defined to allow faster (by virtue of alignment) accesses or more compact storage depending on the system, but it's perfectly possible to write portable code that works with the layout using offsetof and sizeof (edit: and, of course, regular member access :) ).
That said, I would agree that, on the whole, C leans too heavily on under-specified behavior of every variety. It's just not an absolute.
