Framer

A few months ago I asked the sales rep at my local camera store if I could use other lenses on my Fuji and he practically laughed at me, saying "Fuji is known for their glass, why would you put any other lens on your camera."  Since then, I have shyed  away from this topic, but now I'd like to hear what others have to say.  Is it a 'sin' to put a non-fuji lens on our cameras?