I think this is likely true as I read it a lot, but it just doesn't make sense to me. For those who prefer control over their booking what difference does it make if it is done with a TA or on a TA's site versus the cruise line? Control is the same as far as I can tell. I suspect folks fall into this thinking because they have not found a TA that provides value.
I also dont think most of the other half who prefer a TA do so because of advice, guidance, etc. I suspect more, like you and I, it is about the deals available.