The key phrase in what follows is "applied gas". When cask beer is delivered to the pub cellar, it is, unlike keg, a living thing with yeast continuing to ferment and "condition" the beer. In doing so, it produces carbon dioxide and it is this gas, dissolved in the beer, which prevents it being flat and lifeless. The cellarman's task, by careful venting, is to keep a natural level of gas in the cask, but without any excess building up, which would make the beer fizzy.
As beer is drawn off, air enters and progressively dilutes the natural carbon dioxide. Eventually the oxygen in the air will damage the beer by encouraging bacterial growth. The end result is the vinegary pint we all know and hate. However, as long as the beer is sold relatively quickly and the cellarman knows his job, such problems need never arise.
At the time CAMRA started, nearly all local Greene King and Tolly pubs employed this system, known as "top pressure". In Greene King pubs the immediate give-away was the use of white mini handpulls which acted as switches for the dispense spout. Thankfully, most pubs have thrown out the top pressure system, though a handful remain.
The effect of the breather on the taste of the beer is a matter of hot debate. In controlled tests, most drinkers have been unable to distinguish between naturally conditioned and cask breather beers. However there are very good reasons why CAMRA totally disapproves of these devices.
Secondly, having this life-support system on hand often tempts licensees to extend their range of beers. The result, all too often, is six or seven beers in average to poor condition, rather than a couple which are tip-top.
Thirdly the cask breather represents the thin end of the wedge. If CAMRA were to approve such a device, which works by applying gas, it would be difficult to argue against the use of top pressure which does the same, only more so.
For these reasons, CAMRA's recent AGM agreed that in future any pub which uses a cask breather on any of its beers cannot be entered in the Good Beer Guide.
Applied gas isn't the answer to good beer. To achieve that you just need to sell the appropriate number of beers in the right size of cask and apply just one thing basic standards of cellarmanship.