Storm brings a lot to the table. No, seriously. Lots.
Firstly, ignoring her powers, remember Xavier's primary aim is to keep mutants safe and educate them. Not just educate them about the world but also the world at large and foster good human-mutant relations. Ororo was an orphan and then worshipped as a god when Xavier first found her and took her in.
She is noted as being:
... one of the strongest willed among the X-Men, making her highly resistant to psychic attacks, especially in tandem with electrical fields she creates around herself. Telepaths have found it difficult to track her down and probe her thoughts. Several of these traits are independent of her mutant status and are a result of her ancestry.
She is an expert pick-lock and thief, and has training in martial arts and handgun use. She has defeated mutants such as Callisto (superhuman reflexes and a healing factor) and Crimson Commando (cyborg), using her superior strategic knowledge. Storm is also fluent in Russian, Arabic and Swahili. Oh, and she can see in the dark.
To top it off she is from a line of witches; in an alternate universe she became a sorceress of considerable power.
She's a worthy member of his team even without her mutant abilities, and has led the team sans power for a short while, after having defeated Cyclops. (Who, by the way, shoots lasers out of his eyes. She defeated him. Without her powers.)
Talking of her control of weather, it provides attack, defence, stealth and transport capabilities which are noted to be at an insanely powerful level, with some sentinels considering her an Omega-Level mutant.
Sources: Wikpedia
