If I never commit until I'm done you will not see my mistakes, but if there's nothing like `git add -e`, then I won't be able to split up my commits logically and so the upstream history when I push will be devoid of useful detail. Even if I do commit early and often, I will almost never do all my work in the order and logical splits that makes most sense for others to see after I'm done... unless you give me a powerful rebase facility.
No rebase -> no clean history upstream, only lots and lots of merges with pointers to messy history (where authors commit early and often) or shallow history (where they commit only when done) in branches you'll almost never want to see. This is the worst possible outcome! The only logical history organization here is "the large merge commit". You get far too little abstraction on the actual history and far too much noise if you want more detail.
There's also the risk that, because you cannot separate bits of work as "done" and push them separately, your branch will accumulate ever more change and reach a point where you cannot continue without doing much more work, and then you won't be able to easily salvage any of the work done to that point in that branch. This one is a big deal to me too.