This sounds awful similar to the "Nothing to hide argument". People have a right to privacy and shouldn't have to provide a reason why they deserve it. Tracking should always be opt-in.
I mean - most people here use webapps - whether it's HackerNews - or more mainstream ones like Reddit, Instagram etc.
We assume that the webapp developers know when we use their services - after all, it's hosted on their servers.
If you were super paranoid, you could run your own "cloud" services - your own email service, your own document sharing service, your own image sharing service etc. However, for most people the cost/benefits analysis simply isn't worth it.
Apps like Twitter, Instagram, Google Docs, GMail etc are popular precisely because people figure the utility outweights any "fears" they might have over those scary developers tracking their usage.
In a comment above, I provided a concrete example of where telemetry data is useful:
"I worked at a trading company before, and one of the reasons we pushed out telemetry was due to user's feature requests - users would request features, and we wanted to see if they were actually using them, so that we could prioritise developer time accordingly."
Or the same reason most people don't use encrypted email (currently).
I mean, come on - you're storing your data on their servers - there's got to be some trust there.
So my question remains - what is the fear here?