Yes and no. You can find out where your strengths and weakness lie by testing yourself against other systems. It depends how far you want to take your training.
So if you have a stand up only style you may not necessarily have to cross train three different styles to compete in the ufc. You can test against other systems and modify techniques and tactics to compensate.
It is not always about winning. Sometimes it is about finding your limitations.
I think it makes the individual styles more solid that way.
Now yes you could always cross train. And if you want to compete at a decent level you should or do a dedicated mma style. But there are levels that people feel happy with exploring.
Well, I don't disagree with anything you said above, but as it still has very little to do with what I was talking about, I'll have to conclude that we are we're not in the same conversation.