To each his own. In my experience with an Eldar army you can make different builds to seak victory as it suits your taste. For example you can either build to damage your enemy, either buid to gain strategic points, either build to negate strategic points to your enemy or any combination of the above with equal chances to be able to deploy different tactics in each case and build to gain victory.
Out of all the armies the eldar are one of the more flexible, yes. But, try building an eldar armored army. Their vehicles can't do it. Try building an eldar army with high numbers like tyranids, orks or gaurd, again near impossible to do and have it play well. Eldar do not have cheap troops or heavy armored tanks. In a sense, the eldar cannot perform the roll of defender very well. The closest they get is by loading up on wraithlords and wraithguard, but that has its own costs and sacrifice, again in numbers and expensive warlocks and farseers to support the units well.
Likewise, each eldar unit (aside from dire avengers) is over specialized in their combat role. How do howling banshees, striking scorpions or harlequins deal with heavy shooting? How do fire dragons deal with ranged shooting or close combat? How does swooping hawks deal with holding a position? If you take one of these units, it has a single role it fills and it is very limited in its battlefield role, where as a marine unit is more well rounded but not as effective at that single role an eldar unit is.
This is how eldar are put in a role, that their units are meant for one function and are not very adaptable.
Also, eldar are not great at holding objectives in the new rules as most of the units they need are non-troops.