Hello Luca,
The answer is "Yes!" With advanced filters, you can do almost anything, if you think through the logic of the filter.
If you want an All Tasks view which hides the inbox when it's empty, you can use the All Tasks view and add the following Advanced Filters (using Alt-F1 to show the views and filters pane on the left, then expanding the filter details from the bottom of that pane):
In "Advanced" group of the Filter details, click on the "Add advanced" checkbox and then click "Setup..."
Add rule "IsFolder" is false
Add sub-rule
Add rule (which will appear as a sub-rule): HasSubtasks is true
Add rule (the second sub-rule, which will be evaluated with the one above): IsFolder is true
As an expression, this will work like this:
not(IsFolder) OR (HasSubtasks AND IsFolder)
ie, if the item is not a folder it will be visible, or if it is a folder and it has subtasks then it will also be visible.
The <Inbox> folder will only be visible if it has subtasks.
I hope that makes sense. If you play with the Advanced Filters in MLO, you'll find you can work all sorts of magic with your Views.
Regards,
Stéphane