"I was told to avoid registering and hosting at the same company. What do you think about that?"

as

"I was told to avoid registering and hosting at gandi.net. What do you think about that?"

If it was meant as asked than there maybe some truth to it. But than again if you are confident in how some company handles one area than that may give some credibility of their work in other areas.

When I started using gandi they were just a domain registrar. And I am yet to use them as hosting provider.

I agree somewhat with your view, it can be true in some cases :)