I practice several styles, mainly because I seem to have an insatiable curiosity, not because I feel I need to fill holes in one style with elements from another.
In my opinion, it is far better to develop a solid knowledge and skill level in one style first, before training in others. If you try to do more than one as a beginner, you will not make progress. If you wait until you are solid in one style first, then you may be able to progress in two styles at once.
I think this is a very personal thing. For many (most?) people, training in one system is PLENTY. Other people are able to handle more than one. If your one system is being taught by a skilled instructor, and if you are happy with the methods and the training, then it probably offers you everything you need and you should not feel that you are lacking. Don't be in a hurry to train in other styles simply to rack up a list.
My own experiences are with systems that tend to be fairly different from each other. Again, it's not to fill gaps, but rather my own personal curiosity and interest. The methods that the different arts use are quite different, and I simply found it interesting to learn them. My roots in the martial arts are with Tracy lineage kenpo karate. Later, I got into capoeira for about 7 years or so, which is quite unlike anything else. I have also spent about 10 years learning some Chinese methods, including Wing Chun, Tibetan White Crane, and Taiji Chuan. All of these systems approach combat from a fairly different direction, and have different ideas about how technique ought to be applied.
I've found it to be very rewarding to train in these systems. However, It does make it more difficult to keep on top of it all. I have learned a tremendous amount of material, and I do try to work on it all regularly with the exception of capoeira which I have mostly departed from. If I had stuck with only one system from the beginning (I've been training for close to 24 years now) I am sure I would be much better at that one system. However, I have done this with my eyes open, and I felt the range of experience was worthwhile, so I have spread my efforts out over several arts.
I generally do not find myself getting confused from one system to the other. Again, because their methodologies are quite different, it has been pretty easy for me to compartmentalize them and separate them from each other when I am training.
There have been some exceptions to that rule, however. I have recently been learning Tam Tui from two different sources, under two different instructors, at the same time. Tam Tui is an old form common to a number of Chinese systems. I have been learning it from my Kung Fu sifu, but at the same time I've been learning it from my Kenpo teacher, as Tracy Kenpo has also adopted a version of this form. But the two versions I have been learning are not identical. They are definitely similar, clearly from a common root. But they are different enough that I need to really concentrate to make sure I am keeping each version separate.