For the numbers of employees per square metre, its useless - as are all airports. The majority of the land is devoid of work space....
For general numbers of employees - its great. Thousands of jobs in a single location - as are all airports
For location - it's the capital's primary airport, so naturally it has a vast catchment area covering a couple of million potential employees - but how many are capable of working in a high security area?
Remember that Aviation is a global business, and the majority of employees are NOT from this country.
The retail employment opprtunities are the same as any large shopping mall - employment is transient depending on many things including rental rates for floor space. Generally, retail opportunities are limited - particularly beyond custom control.
Extra runways do not directly correlate to employment gains (once built) - more passengers may not require large amounts of new staff.
Any building development is good for employment within the construction industry, but how far can Heathrow expand? Probably not much. Being from the forgotten North, I don't give a monkey's about the exact nature of the extra runway arguement, but a better bet would be to build a new airport entirely. I like Boris's island airport idea rather than extending a fairly old airport.