There is a difference between celestial magic and other types of magic. Celestial is more limited, the spirits already are super powerful and the celestial wizard must train to summon more zodiacs than one at the same time, master moves like Urano meteora, but that's all. As a magic, it's limited, yet powerful if the user is creative. Similar to Yukino's combos or gemini Lucy!
Besides there are people with different strengths and weaknesses everywhere. Ultimately, I think Lucy will play a bigger role than everyone else in fairy tail. That's why for alot she's the main character. Look back where she started and look how much she developed. Look at how she fought her battles and how she always barely won them. Lucy's a character that keeps moving on and tries and tries and will keep on trying to keep up with everyone else. For me she's the strongest character, in her own way ^^
