I don't see the part where he says banelings are better. He says widow mines act in similar role to baneling mines, then talked about a few of the differences, like that banelings are much cheaper, and nonautomated (more accurately not forced to be automated).
Your main point is simply not true. Here are just my first thoughts on ways to harass with mines defensively placed: http://day9.tv/d/ntucker/using-air-harass-against-widow-mines/ And how would attacking be impossible with a widow mine behind a bunker? They have a 40s cooldown, just send 1 zergling in. That isn't hard. People already do this to soak up tank shots. Alternatively, you don't have to attack with zerglings at all. You could for instance use hydras, brood lords, swarm hosts, or many other things that wouldn't get hit by the widow mine.