Using the same username on different platforms only comes with a transfer of reputation and trust advantage if the user can fully prove theyre the same person. I wouldnt trust someone with Theymos as a username on a different platform. Therell have to be need to verify which makes no difference in general.
That's quite easy to do around here. People have staked bitcoin addresses and lots use GPG, so they can sign messages.
For example, this is the *only* way to prove that one is the real Satoshi. Sign a GPG message (his public key has been around forever) or sign a bitcoin message with any of the addresses in the first few hundred blocks and/or genesis block address.
theymos exists on this forum as well as reddit,
https://www.reddit.com/r/Bitcoin/about/moderators/