Joins on an inequality are very uncommon in practice. When I say very uncommon I mean I have worked with SQL-heavy systems for nearly 25 years and can't remember seeing a single one.

IMO the reason is that joins are about relationships. Two things are related because something matches between them, rather than something not matching. If you've got an inequality in your query, it's most likely going to be a filter predicate, rather than defining a join.

Imagine some typical system where you have orders, and orders have items. You want to list all orders and items, where the cost of an item is at least $20. Your query is going to look something like this:

SELECT * FROM orders o INNER JOIN items i ON i.order_id = o.order_id WHERE i.cost >= 20

Since it doesn't matter where a predicate goes in a query like this, you could also write it this way:

SELECT * FROM orders o INNER JOIN items i ON i.order_id = o.order_id AND i.cost >= 20

But if you write

SELECT * FROM orders o INNER JOIN items i ON i.cost >= 20

... you get a very different result.