User blog comment:KnocknickKnock/Celestial combination attacks with Lucy"s 10 zodiac keys./@comment-50.14.119.89-20140330113119/@comment-7418318-20140331010859

Maybe they didn't want to "waste" the time to learn how to summon those spirits.

While we havn't been specifically told, I do think that there's more to summoning a spirit than just holding the key and chanting the incantation.

But even if there wasn't, you still have to know the correct incantation and you need to know the symbols and/or shapes of the keys, so you can say the fitting incantation to the key your holding out.

In addition, as we know from the way her contracts were in the beginning of the series, a fledgling Celestial Spirit Mage can only contract a new Spirit for very few days a week or even less often (like Lyra who Lucy could only call on three days over an entire month).

Unless you have a very subservient Spirit like Virgo, or you invest a lot of time to deepening your connection to the spirits (or alternatively your control over them, in case of the people like Karen and Angel), I don't really see this issue improving. If you can only call the spirit two or three days of the week, it's not all that well suited as a trump card.

While it might be useful for planed attacks, relying on something that may or may not be available to defend yourself is not what most mages would want to bet their lives or freedom on.