A soiled vessel isn't useful until it has been cleaned. Anything poured into an already soiled vessel will also get contaminated. Therefore, neither the vessel nor it's contents can be used to good effect.I guess that's why we need to empty and cleanse ourselves in order to be filled and perfectly used by God. 

"In a large house there are articles not only of gold and silver, but also of wood and clay; some are for special purposes and some for common use. Those who cleanse themselves from the latter will be instruments for special purposes, made holy, useful to the Master and prepared to do any good work." 2 Tim 2:20-21

"Get rid of the old yeast so that you may be a new batch of dough, since you are to be free from yeast. For the Messiah, our Passover, has been sacrificed." 1 Cor 5:7