Wearing a logo is a shortcut. A quick way out for people who don’t have real, established personal brands.
A logo represents something. It’s a collection of associations and ideas. It represents whatever that company’s brand represents, and by wearing it you’re saying “I hope to be associated with the ideas, people, and iconography that this brand and its parent company is associated with.”
There’s nothing innately wrong with this, but be careful, as it’s nearly impossible to stand out as unique when your branding is the same as everyone else who wears that logo.
Like a candle next to a supernova, it’s incredibly difficult for an individual’s personal brand to hold its own when competing with a brand that has hundreds of millions of dollars invested in it every year.
Update: December 22, 2016
It’s true, unfortunately, and I still think there’s nothing inherently wrong with this, but I also think that most people aren’t aware of what they’re actually projecting out into the world when they display certain logos on their jackets or bags or whatnot.
It’s a shame there aren’t more brands that eschew logos, or leave them tucked away inside and unseen. Because another thing a logo often, though not always, does is distracts from the fact that the profit margin on that product is massive, meaning you’re not paying for quality, you’re paying for marketing.