User blog comment:KnocknickKnock/Lucy Stardress:Crux Form/@comment-1776824-20150520200309/@comment-24264679-20150522163318

No where was ever stated to only work on her zodiac keys now it was stated to let her use part of her celestial spirits magic so it makes more sense to assume it works on all her celestial keys. There is no reason why it shouldn't and given how it works all you need is the key. If we use the logic that since she hasn't used it that is doesn't work then by that logic it doesn't work on all the zodiac either just Virgo, Leo and Taurus keys. There hasn't been a situation where a silver key would have been useful but in the entire series there has been multiple situations where Crux and Horogolgium could have came in handy. There is multiple situations when Horologium saves people from life threatening situations and he is a silver key.

The stardress is there to help Lucy improve on her celestial spirit magic and Crux is an expert on Celestial spirit magic his magic is called scanning which lets him scan the celestial database. Stardress is meant to be used as support or back up because she can summon 3 celestial spirits at the same time so why they are fighting she can use Crux stardress to help her gather information that could give her the edge in battle. She wouldn't be open for attacks because she can just hide in Horologium and view the battle. Stardress can be used to support the celestial spirits in multiple ways rather than fighting head on along side them which has it faults also.

Using the stardress she can get the information first hand saving time vs having Crux tell her and magic power. At the same time she can bypass Crux leaving out information due to privacy reasons. Crux can only tell the mage so much without revealing personal information the database holds so he might paraphrase stuff. At the same time Lucy might not know specifically what she is searching for. Also learning stardress doesn't make her an expert.