Because the garbage is not being drawn out of the crankcase it hangs around in there and lots of pretty nasty compounds get absorbed into the oil as a result, and a hell of a lot of water is in there too which doesn't help. The oil life is shortened, and in a Mini its short enough already. Also obviously you are loosing more of it to the atmosphere (and underside of the bonnet!). It will leak more because the internal pressure will be higher. Leaving it open to the air doesn't mean it will maintain atmospheric pressure inside. Its a whole array of oddly shaped spaces in there with tiny openings between them, with the pumping effect of the engine and the heating it pressurises inside and each compartment will end up being a slightly different pressure. Nothing will flow through from one area to the next just because there is an opening. This raised internal pressure will test all the gaskets. Positive Crankcase Ventilation is a good thing, if its not an out and out race car. And as suggested (although tongue in cheek) it is legally required to not drive about spraying oil and harmful vapours all over the place. Its one of the systems and refinements that makes an engine more civilised. If you were building a car that had to be tested in any way (such as IVA) it wouldn't pass open vented unless the engine was pretty old. But its your car, I'm not telling you not to do it at all. Loads of our member's Minis are built like this. As Cooperman says you can use a catch tank to reduce the amount of oil contamination outside the engine.
It's also potentially explosive and whilst it won't blow a hole in the crankcase like it could on a big marine diesel it's better to burn it off safely than let it waft around all over the place. At least with a mini engine the internal volume is always constant, unlike a twin cylinder motorbike with a 360 crank which has quite a bit of pumping going on (felt funny when I typed that, could be the hotpants the girlfriend bought yesterday)....
At least in the mini the volume of gas is only as a result of blowby
