Yes, this was just to point out that there is a similar perception of the necessity on Reddit. I understand you are thinking of a more complex interaction, where the user can join the conversation without leaving their group. 
Maybe this is difficult because a thread removed from the community context can be misleading to someone who joins from outside, so the conversation becomes more complex. 

 Hmm, yeah maybe. I see. 
Do you know if this feature is used often? 
It doesn't seem ideal for the creator since it's extra work during publishing and it's hard to keep up with the discussion in several separated communities. 

 Sorry I don't know, I don't use reddit so much; I know that several communities block them to avoid easy spam.
Btw, crossposting on Reddit is like a repost, so we already have it in its basic form.