How much did the runway extension cost? If you divide that cost by the number of movements that actually need that extension rather than just using it because it is there I wonder if the airport will actually ever get their investment back?
That can be said for just about every runway extension / new runway project in the UK, leaving aside LHR. However, pilots will always say that more runway length is better than less, and extended runways, whether they be at BHX, EMA or MAN (new runway) will assist the respective airports in gaining busines of all kinds (passenger and cargo) which they might otherwise not have got.
One of the problems with the UK is it fails, time after time, to invest for the future, be it railways, roads or airports - infrastructure development has always been to catch up with current demand, rather than to cater for future demand. That's how we've wound up with the cesspit that is LHR.
As for the original assertion that BHX runway can't handle the 380 / 747 because the runway hasn't been made long enough is, I suppose, in part true.
Better informed people than I may be able to confirm, but I imagine that 3000m may still be inadequate for a 380 or 747 at max take off weight to operate from BHX, but that doesn't preclude operators using them on shorter long haul routes such as Dubai, New York, Chicago, Miami. What precludes that is firstly, Emirates prefers to operate the more cargo friendly 777 on DXB, and the market for US services from the UK is overwhelming one for the big twins. Aside of BA / VS how many 380 / 747 movements are there a day between LHR and US destinations. Answer: very few, if any.
For many years, long after it ceased to be true, there was an urban myth around Birmingham that you couldn't fly non stop transatlantic from the airport - even after AA, CO and BA had proved otherwise the travelling public still believed it to be true.
Let's please not start another urban myth regarding the extended runway!