There is serious Considersation on the Back light design for various major Display type of Applications: Not Book (NB), Monitor and LCD TV.
For example Net Book, I think the major concern for Net book is the power consumption or battery life time. The LED display is the major power consumption besides Main Processor board where we can not do much after the CPU Chip is selected with the right speed.
An high efficacy LED usually is low power LED can have a higher efficacy up to 40%, if we can properly design the light guide and filter which can then maximize the battery Life time up to 8-10 Hours.
As for Monitor case, power consumption is no long a big concern, the user is expecting a good contrast and colour saturation, hence we can use direct back lit design. and Cost is the last consideration.
How about LCD TV Back light?
well, all despends what kind of price you want to position for the market on your TV, High end one will be looking at RGB direct back light for best Colour saturation and Highest DCR (Dynamic Contrast Ratio) and cost is the last to consider.
Where as the lower end smaller Size Panel sich as 32" or 42" the Slimmer design, optimum contrast and Colour Saturation with lowest cost will be the best approach for concluding orders.
Hence Samsung major TV series 5,6,7,8 is using Edge Lit Back lit deisgn, only if demanding people will look for 85 series with Direct back lit unit as to pursue best Contrast with customer eagers to pay another thousand dollar for it. Also There is a market need for Samsung to use it as weapon to conteract LG wich is built with White LED back lit unit for best DCR and Colour saturation.