I suppose, just as nice seats will get you higher fares, a high advertising budget will get you higher fares.
Would advertising be simply allocating money or would it involve more control - such as a campaign targeted at a certain city/country/region or targeting a particular demographic. Also, how random would it be - really successful advertising campaigns are usually just because an advertising company simply came up with a really good idea (which is not at all guaranteed). If there is no randomness to it, advertising may become something which everyone/no one does - either advertising will repay itself many times over (therefore everyone does it), or advertising will be useless (therefore no one does it).
While things like seat size and meal cost are quite obvious in how they affect demand, I should think advertising would pose many more problems. While advertising is an important part of any industry, if implemented incorrectly, it would considerably reduce realism, rather than increasing it.
