Well, you have just answered your question yourself. If you know that the recommendation is going to be decent but not great and you already have one required LOR which is great, why would you wait for her and jeopardize you admission chances? Unless you are sure that her letter of recommendation is going to be so great that it may positively influence your admission chances, why would you want to wait for it for uncertain period of time??? To me it personally doesn't make sense...