Sorry, but why would I wear a DropBox t-shirt? If I'm an employee there or something, it might make sense. It also makes sense if you identify yourself with a particular large company's ethos like Apple / Google, but I just can't see myself wearing any of these ...
Why do people wear band t-shirts? This might not be your thing, but lots of people will buy/wear shirts to promote or support an organization they like.
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