When WE are the advertising

We are cheap advertising.  Every time we buy a t-shirt with a logo on it or a bumper sticker or anything that has a corporate name logo we voluntarily become a walking advertisement.  The question how effective is it?  I wonder if people are more inclined to purchase a product when they see their friends or a group a people with it on or in use.  I know in advertising its all about brand image and how many times people see that brand and its quality.  We see advertisements all the time in different facets, but I question their effectiveness.  Advertisements shows why we should buy and gives us reason to buy.  Isn't seeing someone happy with a product enough?  We do not always see others with their products or service and the satisfaction they receive as much as advertisements, but when we do we seem more inclined to buy.