Well, you have to consider the efficiency of using a resistor, compared to using a current regulator. Most regulators operate in the 90%-95% efficiency range, which is possible to achieve if you choose the right LED/resistor/volatge combination. For instance, if you're running a 12V system at 20mA per LED, and each LED (5 in series) draws about 2.2V, then you'll only be dropping 1V across the resistor. That works out to about 92% efficiency. At about 5 cents for a resistor, that's a whole lot cheaper than 14 pounds ($25 USD!) for a current source! It's just as safe for the LEDs, more compact, and easier to build, too.