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Alec Muffet, who helped Twitter on this, replied to your question on his blog: Why offer an Onion Address rather than just encourage browsing-over-Tor? [0] [0]

• anonporridge 5y ago
  > Using onion services mitigates attacks that can be executed by possibly-malicious “Tor Exit Nodes” — which, though rare, are not nonexistent.
  • amelius 5y ago
    Would it make sense to be able to select your Tor exit node, in this case from a bunch offered by Twitter?
    • anonporridge 5y ago
      Then you'd have to trust these Twitter run exit nodes to not attack your traffic towards non twitter sites.
      • amelius 5y ago
        Exit nodes run by Twitter sounds less scary than exit nodes run by random folks.
        • dzhiurgis 5y ago
          Isn't this effectively same as dedicated .onion address?
    • cyphar 5y ago
      That reduces your anonymity almost to the point of being useless. It means that any middle node connecting to a "Twitter" exit node would know you're almost certainly connecting to Twitter (effectively making the three-hop Tor circuit a two-hop circuit). In addition, Tor uses long-term guard nodes (the first hop) in order to defend against certain attacks. This means that the middle node could make a guess about what bucket a user is in based on the guard node. It's not as bad as not using Tor at all, but it gives middle relay nodes far more information than they normally would have -- and middle relay nodes are the easiest kind of node to set up. The main reason why Tor has strong privacy guarantees is because every piece of traffic looks like every other piece of traffic. Changing what paths you pick based on where you're going undermines that in a fundamental way.