User blog comment:IamJakuhoRaikoben/Chapter 394: Juvia vs. Keith, Review./@comment-31.132.65.60-20140725225642/@comment-24066070-20140726012104

Like you said, Silver was waiting for the right moment.

As long as this "moment" didn't occur, Silver would be forced to help with Tartarus' goals (he himself admitted he killed former MC members and "helped" with Face), so I take it Tartarus were willing to let a capable fighter join them in order to speed up their plans while at the same time making sure not to give him a clear-cut opportunity to backstab them.

As for Gray, he is all covered in blood, cuts, bruises and whatnot.

So he may need some time to rest before joining the fray once more and after resting he may need even more time to adjust to his new power, since I doubt he'll be able to use it with the same proficiency as Silver.