I was thinking about this the other day while watching The Pacific
What made me think about elite units was a short piece in one of the episodes where they show a picture of what to me looked like some Marine Raiders. That got to me to thinking about Rangers, Green Berets, SAS/SBS, Commandos, UDT, Spetznaz, and other historical elites and whether they represented a good investment for the militaries that create them. My gut reaction is that in general they are not although there is a role for such special units. Historically, special forces tend to get used for missions other than what they are designed for, thus wasting the the talent and training invested in these units.
By design and their very nature elite units suck the best and brightest soldiers from an army into them. This actually lowers the overall quality of the rest of the military because the best are not available to qualitatively add to the regular line units. While they are undoubtedly useful for special assignments and I would not do away with them entirely, I think the Spec-ops community in Western militaries has grown so much that in some ways they detract from the effectiveness of the line units that make up the bulk of armies. If your best and brightest are all or mostly in elite units then by definition only the second line is providing leadership in the rest of the force. That remainder can be high-quality and I would argue that in the American military at least, this is so. But does that not beg the question of how much more effective an army could be if the Elite community was only as large as mission requirements dictated?
An example of an unnecessary enlargement to me is the Asymmetric Warfare Group created in response to the militant and non-traditional threat in Iraq and Afghanistan. An argument could be made that their mission could be fulfilled just as easily by Delta or Special Forces Troops thus reducing the quality drain on the rest of the force and eliminating duplication of effort.
Disclaimer: I am not down on Spec-ops or Elite units because I tried to join one and failed. For the record, in my 20+ year military career I never attempted to join one nor did I ever have a desire to go Airborne.