Keep in mind that the X-Men team is akin to a team of firefighters.
Would you refuse an eligible new recruit on the basis that you already have someone in the team who is capable of doing everything the recruit can do? Or would you rather have an extra capable pair of hands to fight against the blazing fire when there are lives at stakes?
Comparing their powers individually might show that the others are more effective than he can ever be. Cyclops for example might have more firepower, but that does not mean Gambit or anyone else whom you deem to have weaker power cannot give a hand. After all, in a real fight, you don't always get to pick your champion and have them fight the opponent on 1 versus 1 like in a video game.
Now, consider that people are more than just their mutant abilities. There was this episode from X-Men: Evolution (2000) animated series, titled Cajun Spice. Gambit, who wasn't part of the X-Men team in this show, had kidnapped Rogue. He pretended it was for her own good, to help her fulfil a life of her choosing, instead of being a tool for Magneto/Mystique/the X-Men. However, in reality, he lied to her for his own benefit.
What he needed was her help to save his father.
A similar event happened in the show Wolverine and the X-Men (2009), where he flirted with Polaris (one of Magneto's daughter, who was very shy in this show) to convince her. Although he pretended he wanted to take her with him, as soon as he got what he wanted (Magneto's helmet), he dumped her, because he only meant to use her.
To me, this shows that he knows his way with words and can small talk you into giving in. That can be either considered as charisma or manipulation; something which I hardly saw with the other X-men characters.