Absolutely, if they are open and honest about it and the customer knows who he is talking to at all times.
And I 100% agree with you that it shouldn't be allowed and that models should be required to state that their account is being ran by an agency.
Isn't it possible that you were talking with the actual model and at some point a chatter stepped in. I've seen it once or twice,, like i'm sure I'm talking to the real girl (no unnecessary sweet-talk, short, almost forced answers) and a few hours later it's totally different, full of bullshitting and pushing content.
I guess this is possible. "She" initiated chat again the next day and tried some apology for going off the rails. Offered to prove it again with a pic in real time... my choice of something to write down and hold in front of her face. Haven't had the time to take "her" back up on the offer. We'll see. It's one hell of a process to go through to find someone legit and then, to your point, maybe they are only legit some of the time. While I've disagreed with some on the points on agencies here, I undertand that the models basically have no choice to use them if they want to make any real money. So unless the OF business model changes to required disclosures, it will stay how it is now with continual doubt.