It really depends on your application. If you're fault tolerant, then it's irrelevant. If even a single random error will throw everything down the drain, then you'll obviously want every precaution.
Most web applications don't need that kind of protection, simply because if the page load screws up, well, the user will just hit refresh and keep going.
Well, there's a good chance the same misbehavior or error will happen again and again, until the server is rebooted. Memory errors are not necessarily transient.
You're right, I agree. And again, I'll stress that it depends on the circumstances.
Running a dedicated server with ECC memory costs $$$. Is that client worth it or not? That's only something you can determine based on your circumstances.
Most web applications don't need that kind of protection, simply because if the page load screws up, well, the user will just hit refresh and keep going.