Mesh jackets are becoming more and more popular, I recently acquired one for free by chance. A bike instructor I know had to give up riding sadly after suffering a brain bleed and subsequent fits.
Being an instructor, he had gear for all weathers and his wife has been slowly sneaking most of it out of the house. She offered me the jacket and a pair of Kevlar jeans. Both have become essential items of clothing for me now!
The main advantage of the jacket is just how light it is compared to my normal textile jacket. The massive difference just makes it so much easier to get on and off when you're already feeling hot. The 'mesh' fabric does let in more air but the Versys screen stops a lot of it and warm air doesn't do much to cool you down.
Kevlar jeans do the same although they are heavier and less breathable than normal jeans so the temptation to just throw on your favourite pair of 501's for a ride is still strong.
I've been in South of France for the week and must say I've seen enough riders wearing only a t-shirt and jeans or shorts to say that it's still accepted as part of the norm on the continent.
Maybe the lighter traffic and regular high temperatures make it more acceptable, but I wouldn't like to attend an accident down here!
As bikers we are already taking a risk and wearing anything less than full certified riding gear only increases the risk of serious injury in the event of an off. Therefore each rider must make their own assessment and decide on their own acceptable level of protection.
However, there's also a credible argument to say that overheating on a bike is just as dangerous and increases the likelihood of an accident anyway.
As pointed out by someone above, you can usually tell who's had an off and those about to go down by what the're wearing.
FYI ... I'd recommend a summer or mesh jacket to everyone, coupled with a decent pair of Kevlar jeans.
Next on my wish list is a pair of shorter, breathable summer boots