While flat design done right is good design, especially on the web, I find clients immediately think the work looks "cheap," "unfinished," and "missing life." Seems like many clients still want wood grain and candy buttons. I like this shadow effect, it adds a little more to the design so, perhaps, clients may feel like they are getting more for their dollar if they get those drop shadows that "pop." Wow I sound jaded right now. But I'm not :)

I believe flat design is a solid trend (any design trend with roots in Swiss design are good, especially when organizing content), however these shadow effects are more of a fad that should be used sparingly.