This is anecdotal, but I've found that the more "agile" a place is, the more underworked I am. In most non-agile places, if im out of work its up to me to find a way to make myself useful (agency!), but in agile environments I usually am told not to take something on because "it wont fit in the sprint". I generally think agiles main benefit is bringing the low performers up to average, but at the cost of handcuffing your top performers.