I find that the airlines really cannot impose charges against you (or your TA) if you don't make the return flight. Especially on non-changeable tickets. Because, well, STUFF happens and there are many reasons why you legitimately cannot make that flight and that you cannot change it.

I've had many situations when my plans change and I cannot change my ticket so have to forfeit the return leg. If I can change it even for a fee, I would rather. But then in most cases when change fees allow changes, they usually cost more than the ticket itself. So why bother?