DCL changed their rules several years ago that if a reservation booked without a TA is transferred to TA, their commission is reduced. Usually that means any perks that a TA offers (OBC, etc) is also reduced. Once it's paid in full, you cannot transfer.
If Costco offers a good deal for transferred reservations, I would consider it. Also any changes that you'd like to make such as dining time, adding ground transfers, prepaying gratuities, all has to go through the TA. If you try to call DCL, they won't help you. Some people find that frustrating.
If you have a good TA, it's convenient to have one. If you have a bad TA, it can be annoying.
Our first DCL cruise was through
AAA and they were not very helpful. For example, they called me to say that my cruise documents had arrived and I could come pick them up. Their weekday hours had them closed before I got home from work so I had to wait until Saturday which were only morning hours. Our son had a sleepover planned and dh had to work so I had to pack up the kids on a Saturday morning, including the friend sleeping over, and trek to the next town over to get our documents. (Back then, 2007, it was a big packet.) Not a big deal but after working all week, our weekends are busy. I found a different TA for the next cruise and a few weeks before the cruise, she said I should be getting my packet from DCL soon. I asked if she had to mail it to me (thinking all TAs receive packets for their clients) and she said, "No, I just have DCL send the packet to your house." I was shocked they could do that. Then I realized that AAA didn't bother to do that for me or maybe they felt it was a more personalized service by having me go there. I don't know but I really don't need more to do and would have appreciated if they gave me a choice. I've been with the other TA ever since.
It's not hard to manage on your own. I find a TA convenient especially if I'm using my placeholder for booking a very high demand cruise on the day of release. I can just email her a few days ahead to tell her our 1st choice cabin, or the general area we want to be, and I have a confirmation email by later in the day. I never have to sit on hold for hours with DCL.
If the dates you want are already for sale, it would be easy to manage on your own.
