I'm not sure if there is an "ethical issue" in the original post.

When I obtain a voucher because I have a personal or a business ticket reissued at a lower fare, I am getting a refund of my own money or have saved the original organization reimbursing me for my trip money. But the original higher price had been charged to my personal credit card.

When I go on a subsequent business trip and use the voucher for partial payment, I am, in effect, using my own money. There is nothing unethical about my treating the voucher as a part payment of the ticker rather than as a reduction in the cost of the ticket.

However, if the traveler had been reimbursed for the full price of the original ticket, then use of the voucher would constitute a double reimbursement and that would be a different story. I would think this is the situation that iluv2fly had in mind.