Even with tags, I am constantly wishing I could Visually group my notes into sub-categories that don't require a complex search string to identify.
I prefer to think of the Sub-Notebook more like a Section maybe...
So as of now we have;
Stacks (Multiple Notebooks)
Notebook (Multiple Notes)
but I think I would personally prefer
Stacks (Multiple Notebooks)
Notebook (Multiple Sections)
Section (Multiple Notes)
Tags are ok, but I can't visually narrow down to what I need to from a Tag level unless I create a rather convoluted search.
A very simple example I can give based on a comment I saw (or maybe it was the original description);
Work (Stack)
Client 1 (Notebook)
Income (Section/Tag)
Expenses (Section/Tag)
Client 2 (Notebook)
Income (Section/Tag)
Expenses (Section/Tag)
I could add tags "Income" and "Expenses" to the relevant notes within each Client Notebook, but then the search is [notebook:"Client 1" tag:Income] because the Income Tag will be used across multiple notebooks, across multiple stacks even, or I have to view Client 1 and sort by Tags.
I admit that the Search Functionality in Evernote allows for tons of customization that would allow for this behavior, but the end-user experience should be taken into account, not everyone can set up the perfect search; most people can organize their thoughts by groupings though.
That simple example can get complicated fast once you start adding in other tags that would work better if there were sectioning and not child tags. Income and Expenses could have tags for "Credit", "Rebate", "Donation" and others that would need to be nested either within the parent Tags or set aside as their own tags requiring two tags minimum, now we have [notebook:"Client 1" tag:Income tag:Donation] (I have not used nested Tags yet so my example is based on experience) that must be used instead of quickly looking at the Income Section for Client 1 and picking out the notes tagged with Donation.
Try visualizing this example, and how you might optimize it, with tags alone;
Work (Stack)
Client 1 (Notebook)
Income
Credit
Rebate
Donation
Expenses
Credit
Rebate
Donation
Client 2 (Notebook)
Income
Credit
Rebate
Donation
Expenses
Credit
Rebate
Donation
Home (Stack)
Spouse 1 (Notebook)
Income
Credit
Rebate
Donation
Expenses
Credit
Rebate
Donation
Spouse 2 (Notebook)
Income
Credit
Rebate
Donation
Expenses
Credit
Rebate
Donation
Can you see why a Section/Sub-Notebook/Folder would work better?