As you said, Robin was not. So there are no good reasons to think that this is not gonna happen again.
Well because Robin had a very particular skill that made her very important and Luffy didn't even care about that
Luffy also saved her life while judging her character based on his outside perspective
And then went as far as to break into and destroy 1 of the WG 3 major strongholds and tell her to come back to the crew (asking her to join)
So unless being a lookout is so much of a great skill that it been outlawed or Luffy has judged Carrot as being a deeper person beyond what she has been shown to be
She and Robin's situations are as different as dirt and fine wine
And saying there is no difference means you are intentionally turning a blind eye to the context of why Robin stowed away just to justify the delusion that Luffy will eventually want Carrot, who in his own words are apart of a "bunch of Choppers", to be a part of his crew
Luffy has the final word on who joins and no matter how you want to twist that he is the only opinion who matter