I don't think that there is anything wrong with incorporating different designer pieces into an outfit - I would think that very few people wear the same designer head to toe, and if you did it might actually become a little monotonous after a while. However, I definitely agree with everyone who has mentioned not going overboard on the logo pieces. I typically don't wear more than one recognizable piece at a time, with the exception of my favorite Chanel sunglasses - I wear those with everything!!