After you have fired your agent (for not following through, etc), how do you address your previous representation in your query? If your work has not been submitted to publishers because agent not-following-through never even got you a first set of revision notes?
Do you say you're seeking new representation? Does that send up a red flag or make you stand out in the crowd?
Or, do you just query like you're any other writer and pray?
I think it's worth mentioning that you were previously represented, but parted ways before the project was ever submitted. Why? I think mentioning previous representation shows agents that there has been some interest in your project, that a colleague already showed interest in it, and it makes you stand out from every other query because you're rare and different. And let's face it, the goal of a query is definitely to stand out from the crowd. Explaining that it's before the project went on submission shows them that you aren't trying to shop around something that has already been shopped.