Racedo,
Not entirely the case. I totally agree, original motivation for the tent was a folly. However, the O2 is a huge commercial success and that has benefits for the area and the taxman:
1) All tickets will be vatable at 20% -
afaik even corporate boxes are not deductable but please correct if wrong.
2) Huge range of local companies and support services will be used.
3) The complex also includes cinema, restaurants etc.
4) Knock on benefits from hotel stays and so on.
5) Variety of different events and productions - can't assume all will be offshore or foreign acts. The music industry is an export earner for UK is it not?
See thread about White Elephants turned gold in JB - I think the O2 is one, I can't see same being said about any Spanish airports.