I think you forgot brand ambassadors, people who actually wear your shirt because they love your service, want to promote it, and proudly do so. These ones are the best, because when asked what the company is or does, they'll know what to say and enthusiastically do so. I know bands who wear our shirts and absolutely tell people what Earbits is when asked, which you don't get from someone who says - I don't know, I just got this free shirt at a conference.
Many times companies are annoyed when I say I do not want their t-shirt while at conferences/events. I do not want your t-shirt. I do not wear branded clothing and quality good-looking t-shirts are not expensive.
NewRelic integrated a free t-shirt offer into their signup process, driving leads through the funnel by offering a free shirt if the new trial customer installs the NewRelic monitoring agent. Here's a case study we did on them, 30% increase in leads by integrating t-shirts + marketo + salesforce together using the Printfection platform: <a href="http://www.printfection.com/resources/case-study-new-relic/" rel="nofollow">http://www.printfection.com/resources/case-study-new-relic/</a><p>T-shirts work. Especially for all of these SaaS platforms and tech companies where all of the marketing is virtual. Everyone is doing email and inbound marketing. However, sending out t-shirts in the mail, that's different. People remember the (one or two) companies that have sent them swag in the mail.<p>There's nothing like a t-shirt giveaway via Twitter, or a random VIP thank-you gift that arrives in the mail. It shows you care, because sending a shirt costs a lot more in time + money than sending an email.
The OP leaves out two major subgroups: People who wear the shirt because they (or their friend / partner) is associated with the company, and folks who think the t-shirt is cool or nice. I don't wear t-shirts every day, but if I'm going to wear one, I'm not unlikely to choose a decent one that's loosely associated with something I worked on myself.<p>Lots of startup t-shirts suck, but there is occasionally one that's pretty nice and well-designed. (It helps if you have someone who knows what they're doing design it.)<p>But yeah, if you take some crappy bottom-dollar third-world t-shirt and stamp your logo on it 6" high, don't expect many people to wear it. And too many companies think that's a good idea.
If I get a shirt from a company it's like a coupon for me to be their best friend. Blinksale sent me a shirt after we signed up and I wear it all the time. Logmein sent me a shirt for using a beta product, my company uses them for remote stuff and again, best buds.<p>I'm a tshirt evangelist. If you have a great service, and send me a shirt, I'll rep your stuff and pass referrals. But I think the key is that it has to be personal. I won't do it just for a shirt, I'll do it for the relationship and service included.
I wear my stripe CTF shirt a lot because it's super comfortable, doesn't look absurdly terrible, and was a reward for doing something cool. So i'd recommend that.