I can't think of any reason not to use a TA - Unless you have a bad or crooked one. A good one will be an advocate for you and look out for your interests. Heck, even if I've done my own research I still use my TA. They have always went to the bat for me if needed. Even if I book on board the ship for a special etc., I have them assign the booking to my TA.
I've often heard people refer to the cruise line's various names for their respective sales representatives as agents. They are not agents, they are order takers. Their loyalty is with the cruise line not you as a guest. A TA is just the opposite.
Just my 1.5 cents.