Let's try and not turn every flaw of every Telegram app into the OMG-look-at-these-morons fiesta. This is an issue with a specific version of one out of many apps, not a protocol issue and it hardly warrants the near-hysterical "NEVER" in the title.
If you are a political dissident and use the app, you might get jailed or killed due to the fake sense of security. Secure IM is not for hiding your diary from your sister any more.