I would assume there to be some benefits to carts over wheelbarrows the way Toady implements it, we just don't know. Apart from that, yes, maybe you are right, maybe any true efficiency increase comes from the implementation of hauling things in bins, whereas carts are just the bins stones use and are more for novelty. It is difficult to say at this point though without more information, so why not pose this as a question to Toady rather than being so pessimistic about it? Although to be honest it might be more sensible to simply wait for what the devlog updates over the next few days reveal.
As for your concern that dwarves might not use the carts etc. to their full potential by not filling them up: even if there is no way for them to wait (which might be implemented in some way for all that we know), as long as items pile up at their point of origin there should be an improvement. You say that workshops produce constantly and dwarves haul constantly, but whether items pile up or not up depends of course on the number of haulers assigned. If the new systems works, then you can assign less haulers and allow for produced items to pile up, temporarily that is.
There's nothing wrong with a little pessimism, so long as you keep perspective.
I remember seeing several threads around when people were expecting older major releases to come out, in which posters were encouraged to guess what the new bugs would be for the new features going in.
Something like that might seem rude at first glance, but if Toady actually just read that, and looked at it as a checklist of "make sure this doesn't happen", it would actually be quite helpful.
Stress-testing an idea before it is fully implemented (and hence, changes are cheaper and easier to make) is more useful than play testing, which is much better than just leaving the bugs in there.
So never shy away from a good pro vs. con argument.
Anyway, yes, from what the devlogs look like, this will be more akin to a "train station" setup, where carts merely have stops, rather than are capable of being moved anywhere the rails allow.
That could potentially allow for a "loading platform" that acts as a stockpile, itself. If the objects that are meant to be shipped "miss their train", they just sit in the loading platform until the "next train comes in".