Well, I would have liked to have made every book in the series stand alone, which I tried to do with plenty of explanatory stuff in subsequent books, but there comes a point when there's too much of that, and too many plot components becoming very complicated. Macmillan reluctantly started numbering the books at number four, Polity Agent, and I believe future printings of the other books will be numbered. The reason they weren't numbered from the off was that I didn't start out with the intention of writing a series -- that idea took root at about the end of The Line of Polity.
Muscular, intelligent and often amusing? How could I object to that?
Yes, readers of Banks might like them, specifically readers of earlier ones like Consider Phlebas, where there's more exploding spaceships and homicidal aliens!