As someone who practices a couple different systems, including Wing Chun, I'll give you my thoughts on it.
I do believe that if you are really proficient with any art, you probably don't need to study many different arts and mix them together. This doesn't mean that every art contains every element of combat to an equal degree. Far from it. It is very obvious that most arts have a stylistic approach to combat which focuses on specific methods, altho elements of other methods may also exist within the system. Meaning: an art that focuses heavily on striking and kicking as the favored approach may still contain some throwing and grappling and joint locking methods, altho they may not be as thoroughly developed as an art that focuses on these methods primarily. But of course the second art may lack in the striking and kicking area.
However, if you gain true ability in any art, I believe that you ought to have the tools necessary to handle any kind of combative scenario. Just make sure you fight YOUR fight, and don't let someone sucker you into fighting HIS fight.
As for myself, I practice several different arts simply out of curiosity. I seem to be cursed with a never-ending perpetual curiosity and I just can't seem to help myself. I find it fascinating to explore the different approaches to training in martial arts. I do not practice different arts to plug "holes" in the system, so to speak.
That being said, I feel if you do practice more than one art, it is best to practice them separately from each other. Different arts are built upon a certain base, which makes their specific techniques work. If you try to graft those techs onto a different base from a different art, they often do not work as well or even fail miserably. This is not always true of course, some things blend well, but in general I believe it holds water.
So practice them separately for what they have to offer individually. Don't mix them up and build a hodge-podge style of mix-and-match stuff that doesn't function well together. But, if you ever need to actually use your skills in a fight, you can switch up your methods and use whatever is most appropriate under the circumstances.
As a side note, my martial origins are in Tracy kenpo. Years later, when I was studying wing chun, I did find elements of my kenpo techs creeping into my chi sau practice. It seemed to me that some of that material did in fact blend pretty naturally and efficiently.