I completely agree. I'm fluent in four languages: Spanish, Catalan, English and Japanese, the first two native, plus I can understand (but not speak) Italian, Portuguese and French because they aren't that different from Spanish and Catalan.
I can see how at least in my case, learning how some constructs work in one language has helped me understand things about another.
It's also true that the language used to express yourself in a given situation affects the way you think about it, because of what you can and can't express, and the limits each language imposes on how you can communicate.
Often, when I talk to my bi/trilingual friends we find ourselves changing languages mid-sentence and speaking in a mix of them, not because we don't know how to say something in English or Japanese, but because sometimes it's either easier or more accurate to use one language over the other.