no left work on closed

Even if it is possible to set done status with left work (it means task is complete, but there may be some extra work to do, such as validation, delivery, ....), it is not possible to close item with left work. You MUST set left to zero before.

it seems to be a bad practice for project management.

I completely disagree with you : it is a great practive as left work is key value for project management

Once the activity is completed, the estimate time is only of secondary importance.

YES, abolutely.
And you'll see that progress never takes into account initial work.

The project manager may underestimate or overestimate the workload

Of course, he is not a seer.

this should not prevent him from closing an activity

Of course it should : if there is left work ,it means work is not finished

As you may know, there are performance indicators for project managers that measure the gap between estimated time and real time

YES, and PM should not change initial work (validated or assigned). But LEFT WORK should be changed...

Do you think that we could one day be able to close an activity with left work?

ASOLUTELY NOT !!!
We have also recently add feature that allow PM to forbid to "end" (mark as done) items with left work.

YOUR ISSUE, IS THAT YOU NEVER WHANGE LEFT WORK.

Resource working on a task should revaluate left work at each input, to give the information to the Project Leader that estimate was under or over estimated.
This can be done VERY easily on Real work allocation screen.
Resource should set left work to 0 when work in complete. There is also an altomation that can automatically more activity to next "done" status when left work is set to zero.

And if reosurce does not do the job, it's PL duty to change left to zero on activity screen (very easy since V6.4 directly on assignment table.

So NO, we will never allow to close items with left work
But you have some chance : in V6.5, we added a helper called "force close" as a specific access rights, that can allow some profiles to close items with left work : the left work will automatically be forced to zero. IMO this is a very bad practice, and it is here only to be able to close a project that is sudenly stopped.