I don't see any drawbacks of this implementation if it were to be implemented?
How about this drawback: People would use it.
We're talking about chat here. Conversation. Talking. You can't go back to last weeks party, choose the things you never should have said, and un-say them. In particular, that could quite well remove any sense from the rest of the conversation that was not deleted.
If you feel strongly about it, go ahead and flag for a moderator, but in the usual case – just let it go! It's not the end of the world that there's a mis-formatted message somewhere.
As I side note: As a moderator, I'd much rather see the occasional "I screwed up the formatting here and it looks bad, could you please do X" flag than those ridiculous "This is off-topic!!!!" flags that keep coming up because someone dared to mention in the PHP room what they had for breakfast.
And regarding your comment on Pollyanna's answer:
2mins isn't exactly much time to review a code block
That's true. And the chat isn't an IDE.