I know from your perspective that makes sense - However, some agencies literally have rules stating that if a client from another sales agent within the same firm approaches someone to take over a sale, then they must approach the original sales agant first, and some agencies will go so far as to state that the secondary agent will not make contact with the sellers again without the orginal agents express consent. Even without those rules, it is professional courtesy and loyalty for a collegue (and helps to avoid all sorts of legal mindfields in relation to commissions, etc).

Given how useless she has been for you, I would look at giving her the flick too. However, your first port of call should have been to inform her of your dissatisfaction and that you were intending to look elsewhere for someone elses services.
You may find that even after you give her the flick no-one else in the same agency will take you on as a client, because there are still legal issues that may very likely arise regarding commission.