Think that the point is BMW wants to keep the baseline 5 an in-line 6 and that the only way of 'improving' on the base 520 is to have larger capacity and make the engine more torquey.
Thus like yky pointed out above, we had the 523 in the past which was a 2.5 detuned. This time, you have it badged 520. Just not sure whether the 520 engine will be a valvetronic but it seems foolish not to have the latest tech in their baseline petrol engine 5.
Alan,
Your point about the 320 is a good one. In future, you will have to cough up a pretty penny to gain entry into the inline6 club with a 3 series.
But I think our prior mental barriers with 2/3 and 4 litres regarding road tax has changed. With the exception of cars above 4 litres, the jumps in roadtax for the big figure change from 1-2 and 2-3 litres has gone and people's mindsets have changed. Just look at the number of 330s on our roads. In the past, the E36 325/328 was much more rare.
If I were in the market for an E60 next year, I would definately get the 520 assuming of course PML prices the car competitively vs the 525 like it currently does.