I do think your associating with seasonality is interesting and I think I get where it is coming from, but I personally wouldn't quite go that far because I have seen unstructured winter suits that look really nice as well. So in this case I can see why a summer suit might be better with less structure in terms of coolness, but on the other hand it's common for winter stuff too.
I didn't mean that all winter tailoring should/must be structured, I'm well aware that there is plenty of examples that prove otherwise. It's just my personal feeling, that shoulder construction gives a more natural vibe when it comes alongside winter fabrics, colors, etc.
Having that said, I think that 8 out of 10 times, I'd go with jacket with little padding/structure regardless of the season it belongs to. But I guess it's a matter of personal taste.
My experience is quite different, but I guess it's due to my job spec (I'm an IT consultant) where the dress code is non-existent and sometimes I get the feeling that I'm expected to wear black t-shirts and jeans all the time even after moving to an investment bank
I've also noticed that a lot of the people who wear suits because they are forced by dress codes in their work environment, tend to don't care about the fit, quality and other important things when it comes to their tailoring. The best examples of classic menswear I've seen are from guys, who have great knowledge about it and treat it as their passion. And they are often suited-up also after hours - hence the idea of casual suit comes to me quite naturally.