The No Vent has it's origins in that land of eternal style and fashion, Italy. The Italians have a remarkable eye for the detail don cha know and this is why they got rid of the vent in the first place. Ultimately the vent is utilitarian and detracts from form! In other words vents are useful but a look rubbish! Taking away the vent left the suit looking streamlined giving the illusion that the wearer was not only taller but had a touch of elegance about them. The downside was that when the chappie sat down or put his hands in his trousers the blazer would ride up at the sides causing a bunched up look which, over time, caused creases, which lead to frustration, leading to anger and then global nuclear holocaust and we don't want that now do we.
Supposing a policeman were to creep up behind you and grab you round the neck? What would you do? Well blasting out a couple of hot cubic litres of gas would be pointless wouldn't it as it would be blasted on to the floor! None of it would end up in the face of PC Plod would it and he'd be at liberty to do what he damn well likes NO SIR! For that scenario you'd need a single vent.
And there is another advantage over the no vent here too. If you find your situation getting a bit sticky with your assailant you can either bend forward just a tad or put your hands in your pockets thus causing the vent to open up just enough to give you that extra thrust necessary to tip him over the edge.
And there is another benefit here too, as the vent parts the blazer falls a little to your sides thus protecting innocent civilians to your left and right not just from released gas but also from exposure too should your trousers have been dropped. This is important! The last thing you need is collateral damage in the form of innocent bystanders.
Oh and the single vent is more popular in the United States and Australia than in the UK, it was a variant developed for people riding horses and facilitated the blazer to drape either side of the rider's legs whilst saddled. It is slightly cheaper than the double vent for this reason.
The Double Vent
However, all is not lost because if you are accosted from either side of you then you are in a winning situation with the double vent. You see the blast will invariably fire out of the side vents taking both your attackers down in one blow! Marvellous. I've done this myself on at least two occasions with stunning effectiveness. Both men were rendered completely incapacitated for 45 seconds giving me time to call the rozzers!
Word of warning here, whilst releasing the wind, it is imperative to remain standing upright. Bending over here will cause the sides of your blazer to hang downwards thus exposing potential arseage to the general public. Usually, if well trousered, the public can handle betrousered arse without batting an eyelid but do keep this in mind.
Oh yes and the double vent prevents creasing, covers your backside when you put your hands in your pockets and if you've got a fat arse it won't show as much.