Sadly a lot of salons mix this that and the other.
Using systems isn't a gimick that the nail companies put out, it the best way to find success! You get the best product performance when you use a system and you have the technical help of the manufacturer should you ever need it.
When I first trully comprehended the importance of systems and started moving toward being systemized there was a huge improvement in my nails!
There may or may not be an adverse reaction when you cherry pick or mix and match, but why stake your professional reputation on it? I think my thoughts may have wandered about a bit sorry, I just believe in it so fully I can hardly express it!!