(1) It is easier to get a better resolving ability with an EF-S lens instead an EF one when used on APS-C
(2) "Get the most out of the 18mp crop sensors" is just non sense. Resolution will improve when you improve either the sensor or lens. So, a better lens will improve the end result resolution on both a 12mp and an 18mp sensors. On the other hand, an 18mp sensor will improve the end result resolution compared to a 12mp sensor, even when used on an old low resolution lens. So the 18mp sensors resolution with the current lenses, are ahead than the lower mp sensors and don't need any extra help, while an improved lens is welcome for all sensors not just the 18mp ones
IMHO a good lens will highlight the defects in a cheap sensor and a high mp sensor will show up the flaws in a cheap lens. Therefore, if you've got a decent crop camera, you want a damn fine lens.
An image produced by a crop sensor has to be enlarged [eg. x 1.6] more than a FF image just to produce the same size print. You enlarge the image... you enlarge the flaws. That's why, traditionally, large & medium format lenses don't have the resolving ability of smaller formats like 35mm. They don't need it.
What I'm trying to say is that, if you're using a crop camera, it's probably more important to have excellent quality glass, and therefore L series lenses are a no brainer. I think that's what the author of this thread was trying to say when they said "â€œLâ€ lenses and the development of them are whats needed to get the most out of the 18mp crop sensors."