Maybe if you live in a cold climate, then wearing a long-sleeved shirt and trousers won't be a problem. In Sydney, where I work for a company that permits casual dress, most of engineers show up in shorts, t-shirt and flip-flops for the whole summer. The formal suits that I see the bankers wearing to work are neither well suited to our climate, nor essential for the work most of them do.
When I walk to work in a t-shirt through a city of suits, I mostly think about how it's the freedom I like. Freedom to wear appropriate clothing, rather than being tied into wearing something to fit in, or to impress somebody.
Fair enough, although in that case I would wear linen trousers and a short-sleeved linen shirt. That combo works in Spain, Italy and North Africa in summer. Short-sleeved linen shirts feel really cool in hot weather, really nice.
You absolutely shouldn't wear clothes you don't feel comfortable in. However, wearing a t-shirt because it's an act of rebellion doesn't work for me at all. If you're wearing t-shirts because you want to, then fine.
Wearing smart clothes is not something I do, or my colleagues do, to fit in. I work in a university and often we are the only people (6?) in a department of 200 people under the age of 60 in formal clothes. They feel comfy and they look good. People also find them intimidating, which can be funny.
So long as you dress how you want, that's cool. I just find the idea of wearing free, logo-infested clothes a bit unappealing. I come across an attitude of 'I wear t-shirts because I don't have to wear a suit!' pretty regularly; I find it sad because it's being constrained in your dress by those standards in the opposite way to the suits but not even acknowledging it! Wear what you want!
It's even funnier for those of us living in the tropics (Darwin, in my case). From where I sit Sydney's summers are pleasantly cool and dry.
"Territory Formal" is a short sleeve shirt and trousers. No jacket, no tie, no long sleeves. It is just too damn hot and humid.
I can usually spot "southerners" in the CBD: they walk one way up a street wearing the jacket and tie. They walk the other way, beet read, with the tie loosened, sleeves rolled up and jacket over their arms.
Especially in hot weather I actually prefer short-sleeved shirts to t-shirts. The tightly woven, thin fabric of a shirt does not stick as much to the body as a t-shirt.
Though I have to admit, that I have never been to Australia in summer. And a suit would definitely be out of the question for me as well during the hot season. :)
"Australian summer" is a very broad term, given the size of the continent. I'm in the tropics, for example, and let me tell you: everything sticks when it's > 30C and > 70% humidity. You could make a shirt out of teflon and it would stick.
When I walk to work in a t-shirt through a city of suits, I mostly think about how it's the freedom I like. Freedom to wear appropriate clothing, rather than being tied into wearing something to fit in, or to impress somebody.