I don't agree a person wearing a Metallica t-shirt is actually promoting them. It's more of a case of saying, "Hey world, I'm cool because I like Metallica!". It's a case of showing off your interests to the world, not an altruistic promotion of a particular interest.

"I'm cool because I use a relatively obscure software tool to backup my data" - doesn't sound right.

EDIT1: Also I'd rather have startup quotes presented in an artsy way than just logos like this - http://startupquote.com/

EDIT2: Other cool ideas instead: @adrianwaj and @bomatson