The information you got from your local homebrew shop is correct. If beer in a keg is exposed to the atmosphere, the carbon dioxide in solution will migrate out of the beer and equilibrate with the partial pressure of atmospheric carbon dioxide. The question comes down to the definition of “flat beer.” If the beer is stored cool, say 55 °F, with no top pressure, it will contain approximately 1.55 volumes of carbon dioxide. If the temperature of the beer is increased, the carbon dioxide level will drop even lower. To put this level into perspective, most American beers contain between 2.5 and 2.8 volumes of carbon dioxide. Beer with 1.55 volumes of carbon dioxide would be considered totally and unarguably flat by the vast majority of American, German, Japanese, Brazilian, Australian, South African, Mexican, Canadian, and Bahamian beer drinkers, just to name a few. However, if you take that same beer, squirt it through a sparkler attached to a beer engine, and place it in the hands of most anyone from the United Kingdom, it would be considered perfectly normal. If your goal is to homebrew a nice ale and serve it from a beer engine in a traditional manner, then the CAMRA cellarman’s guide is absolutely correct. Since the English drink beer qualifying as flat, it takes a very long time indeed for it to get any flatter.
In all seriousness, cask ales usually contain roughly 1.5 volumes of carbon dioxide and are stored near cellar temperature (55 F) so they are already more or less in equilibrium with atmospheric conditions. Some cask ales contain a little more carbon dioxide, somewhere between 1.55 and 2, and will lose the little carbonation they have with time. The concern many have with the CAMRA guidelines of leaving the cask vented to the atmosphere for prolonged periods is beer oxidation. This not only begins to change the beer’s flavor, which some people like in a cask ale, but the oxygen allows for the growth of aerobic beer spoilage organisms. These bugs only grow in oxygen-containing environments and are not a problem for beers dispensed with carbon dioxide.However, cask beers can and do spoil due to aerobic bugs. The most notable example is spoilage from Acetobacter species, which convert ethanol in beer to acetic acid (vinegar). A slight over-pressure of carbon dioxide or a mixture of carbon dioxide and nitrogen will prevent the problems associated with oxygen in the headspace of your keg. Care must be taken not to use too much pressure as the beer will over-carbonate just as it will lose its carbonation.