I'm the same as you, I've always booked all my Disney trips myself and have never used a TA. I did use a TA for my cruise to get the onboard credits.
On my last cruise I booked a 7-night on the Fantasy with a verandah room and got $200 in onboard credits from my TA plus an additional $25 in onboard credits from Disney. On the most recent cruise I booked, I went with a 4-night on the Dream with an inside room and got $50 in onboard credits from my TA and an additional $50 in onboard credits from Disney.
So yeah, just having that additional money to spend onboard on excursions, at Palo, on drinks, on shopping, etc. is definitely worth using a TA!!! And it doesn't cost you anything to use a TA!
I use
Dreams Unlimited (which owns the DISboards.) I've had a fantastic experience with them! And I liked it because they pretty much just booked my trip for me, got me my OBC, and then I got a log-in for the
DCL website to book all my excursions, dining reservations, spa reservations, etc. all by myself like I prefer doing. I never had to tell my TA my requests, I was always able to just do it all myself.
But my TA was there to answer any questions for me if I had any or to help with anything I needed help with.
Now, I can't speak for any other TA because I've only used Dreams Unlimited, but I thought it was the perfect combination of doing things yourself when you want to, and having your TA help you with things when you want them to.