I would say those DOCs were a bit out of date. I have just had this very conversation with the operations director of the company I work for (we fly B757).
The answer was the lease costs of the 757 are significantly lower than the 737-8/900, a B757 can currently be had for about $80K a month (£45K). The fuel costs and over flight charges are higher, but can be kept under control by use of lower declared take off weights. The big down side with the 737 is the size (180 seats is A320 territory) and the lack of payload. Cargo is starting to be come big business for the charters again. Or for mine anyway.
The charter companies in the UK and USA are putting pressure onto Boeing to build a narrow body version of the 787 to replace the 757. And Boeing are looking at it. We are looking to keep the 757 until 2012.