There maybe some people who want this but they are not the marority. A task and an application are unrelated things. A task most likely spans one/several browser tabs + a couple of documents in different apps.
I haven't got into the habit of using them but the original issue we're talking about is arbitrary grouping that does not seem to help how people work in reality.
This is actually the first time I hear a user making this argument. Aaron J. Seigo would love you, as he added Activities in kde 4 for this exact use case.