Dear Shahran,
Please understand our view, we always try to deliver what majority of our users want. And we try make the design in such a way, that it fulfill the needs of all.
In this case, the problem was billing based on old timesheet. Because timesheet was designed for tracking activity for a single day. But billing needs selective activity (time logs) from multiple days, which was not possible using time sheet.
Regarding usability, we have provided calendar view of time logs, to cover up grid structure entry system of timesheet. If users forgot which they have entered earlier, they can easily check calendar view.
Maintaining multiple versions are against our work culture, it adds extra complexity and needs more resources to manage the product. We already have comprehensive to-do list and shortage of resources, we really can't maintain different versions of same feature.
P.S. We will surely rethink about the design, if most of ERPNext users think in the same way.
Regards,
Nabin Hait