Lists are fun. For my money, the three 'Big Cs' - camaraderie, comfort and convenience - capture much of the essence of why running shirtless (or jogbraed) is an attractive option. But let's break things down to nine reasons to go bare:
1) Your body cools more effectively: No wicking shirt will do the job as well, whatever the promotional literature may claim.
2) It ensures you stick to your exercise regime: If you know there's more of you on display, you'll make sure you won't cut too many corners.
3) It saves on laundry: If you play tennis three times a week, or run four times a week, that's 12 or 16 shirts saved respectively from the wash.
4) Enjoy the brotherhood: When another shirtless person hoves to view as you run, you feel an automatic bond. Honest to God.
5) It's just ever so slightly naughty: In the most harmless way imaginable, the shirtless runner bucks anoynimity (though if enough people come on board, we'd be the majority, which would change the dynamic somewhat. For now, however, that's not an issue).
6) For those logging serious miles, you avoid nipple-chafing.
7) Get an even tan.
8) Up your speed: It's been attested to by many runners, though it may not apply to everyone, that there's more zing to your step when you're shirtless.
9) Lead the pack: It's the same attraction that draws some folks to be early-adopters of new gadgets. Anecdotally, plenty of potential shirtless runners are held back by reserve and a lack of exemplars. There's nothing like setting an example and seeing someone else gain the courage to turn up in jogbra or sans shirt.
* I've stolen some of these thoughts from others, without remembering who's suggested what. If anyone's got other reasons, send them in and I'll add to the list. *
May 2015: First up, though I still try to put up blog content whenever I can, it has been easier to more regularly visit the the Twitterverse. Follow me at @barethomas10 and let's keep the shirtless running flag flying. Of course, the blog still attracts very interesting comments, and good discussion. Keep it up.
Second, in the years since this venture launched, and as shirtless running among women has gone increasingly mainstream, the term "jogbra" has clearly declined in use. I will thus prefer "sportsbra" henceforth - as has already been the case on Twitter, and in recent posts here.
I continue to welcome guest posts (sent to email@example.com) on any related topic, including from those who would discourage stripping to the waist. I am myself of course a fervent convert to the joys of running bare. But let all voices be heard!