I think there's a difference between being the weakest and being useless.
There's definitely a massive difference between the two. Sokka in Avatar the Last Airbender is a good example of someone who was certainly weaker than his companions, and this notably never changed, yet still proved to be a valuable ally at key moments and became more useful over time as he actively set out to improve on his fighting capabilities. Usopp over in One Piece is another great example.
Kenichi, from The Mightiest Disciple is also something of an example, as he was the weakest person at his dojo for the longest time (And arguably might have always been the weakest), with various masters being capable of ending the majority of threats that popped up in an instant, but the story was written in such a way that the threats were his to face and therefore his master's strength never entered into the equation.
A character being weak doesn't mean they can't be interesting when the writing is solid.
I can't force you to believe me that's your choice. People live their lives bound by what they accept as correct and true. That's how they define "reality". But what does it mean to be "correct" and "true?" Merely vague concepts...their reality may all be a mirage. Can we consider them to simply be living in their own world, shaped by their beliefs?-Itachi Uchiha