Among other things, the difference is speed: VALE can do 18 Mpps versus the 0.8 Mpps of a Linux bridge. And sometimes you do need speed, maybe not for virtual machines but e.g. if you want to stress test your network monitors, routers, IDS etc. For instance, using VALE we found out a missing event notification in QEMU/KVM which limited the receive speed to 50 Kpps, and a one-line fix brought it up to 1 Mpps.