You can look at this in two ways:

a) If a customer has taken the time to draft a letter to senior management then the customer could be giving an early warning of an issue which, if not nipped in the bud, could escalate into a serious problem for the business.

or

b) It is not the job of senior management to micro-manage the business and deal with individual customer complaints.

If there is an increasing trend of customers writing to senior management, then the concern for management is not those specific complaints, rather that customer services do not appear to be handling complaints adequately or that, as is common in organisations, there is a filtering of information up the management chain and management is not being made aware of customer concerns.

There are only so many hours in the day and, particularly for the airline industry, there are host of other organisations and interests that senior management have to deal with (government, the media, regulatory bodies, alliance partners, the list goes on and on). The organisation should be able to take care of the day to day running of the business itself and the job of senior management is to be looking ahead at the major strategic issues facing the business.

I can see both sides of the argument. Though I do think if a customer has written to Martin Broughton or Willie Walsh, then to receive a holder letter promising a reply and then nothing is unacceptable.