WAN can be classified from your branch to HQ or POP , or maybe to the data center or even between the data centers. So depends on the traffic , sending them to different links is very common. For example between the data centers , backup , VMotion or any other data center interconnect traffic is sent from one link and the latency, jitter and loss sensitive storage traffic from the others.
Or from branch to HQ or DC traffics. Latency, jitter and loss sensitive real time traffic might be sent through MPLS but less critical WEB traffic ( depends on business requirements ) might be sent over DMVPN through Internet and so on.
So flow based forwarding or classfying traffic at the edge and send them over the links which supports the traffic requirements are commonly implemented through DSCP marking , PFR , OER , MPLS Traffic Engineering and so on and What Greg says are important. He mentioned that we need also simplicity and SDN will promise to bring it. Definitely SDN will find a more use case on the WAN and Google and other big enterprises already use it over the WAN.