I feel that most systems offer something but ultimately it is up to how it is presented, practiced and how the student chooses to absorb it. If you teach it like it is simply a way to stay healthy and the combative/defensive aspect does not appear and even if it is, is it realistic? From the student perspective you can also have a student who is studying it just to study it and regardless of how it is presented they would not be likely to use it effectively as a form of defense. You can also have someone who can use tai chi in the park to wreak havoc.
I have had the opportunity to experience a wide variety of systems. Two that come off as less than stellar (to me) are Budo Taijutsu and PPCT. My X-Kan was always instructed as being "way to dangerous" to spar with (okay
) and (again based off of my experience) PPCT always seemed to breeze over the combative(s) portion so that the pressure point portion of the system could be over emphasized.
So to reiterate I think Wing Chun is great and it can be a great combat system but there are a lot of factors that take into account beyond just the system. I think MMA is great but in a practical, outside the ring situation maybe no so much.