Sure, but "most people" aren't going to actually have the data collected on them examined. Just rising young party stars, perhaps. Or civil rights and environmental activists. Or the CEOs of companies that are competing with friends of the head of GCHQ. Do those people talk about banal things? Do those people talk about illegal things? Do those people talk about embarassing things?
Honestly, the issue here isn't "most people's" banal talk. The effects of having such a huge cache of data on hand to get "leverage" on key people that the GCHQ doesn't like is the issue.