"On a machine like a dirty fan, havign a ruined bag must ruin the performance completely!" Well there is that, but even more serious would be with a clean fan (such as the Convertible) that a re-used bag will restrict air flow (and thus motor cooling) and could also burst.
Now I know Hoover produced H4 and H18 "twice use" bags, but this was simply to appease people who may have been put off buying or maintaining a bagged machine due to the price of buying bags. The fact they could use one bag twice would have halved their running costs and "sweetened the pill", so to speak.
Indeed it worked and many people used the re-usable bags more than twice, often into double figures even (my 1994 Turbopower 1000 had a twice-use bag in it when I got it which looked to have been used for years). Needless to say a couple of years into my ownership the years of neglect took its toll and the motor blew.
As I stated above, once a dust bag has been filled to half way the pores are already clogged up with dust and although emptying the bag will restore suction, this is to the detriment of the motor as it will be having to work very hard to counteract the impeded air flow. Much like if you load a car up with heavy cargo then floor the accelerator - it will still accelerate well but the engine and transmission will be working terribly hard to counteract the weight.
Of course Hoover knew exactly what they were doing, and that people would use the bags more than twice. This was actually very much in their interests as more broken down vacuums mean more people buying new Hoovers, or at the very least getting them repaired by Hoover contractors.
In every sense the introduction of clean fan vacuum cleaners has been a godsend for manufacturers as - by their very nature - they wear out much quicker than dirty fan cleaners (on which having a full or re-used bag does no damage at all, other than to impede performance), but they are also much more practical so consumers will happily buy them all the same.