We used a TA, and I'm SO glad we did since it just made it all easier on us and our guests. Any guests that book through your TA count towards the credits, but if you have anyone who books elsewhere, it doesn't count. I think it saved a lot of stress of people not contacting me about things, and I had a lot of delays in communication while working directly with BP that were a minor hassle for me, but if all of my guests went through that per my suggestion to book through them, I would have felt really bad.
We had some guests with some room issues when they first arrived that were all taken care of by our TA. We also had four different guests who had their flights get canceled on the way home, so since they had a TA, she took care of getting them on new flights and everything, so they didn't have to worry about it.
And not to mention when I was having a difficult time getting answers prior to signing the contract and getting everything confirmed, our TA bugged BP, and we got answers immediately! It's good to have someone outside of the hotel on your side.