Interesting you raise this issue, I have had a near identical situation. Etihad doles these vouchers relatively happily to passengers who have been denied boarding for whatever reason, knowing that these vouchers are very difficult to use at out-stations.
There are two points here, one is that amount that they give you is not enough, but I won't get into that.
Second point is usability of these vouchers. I tried using them at JFK and was told they can not be used at the airport, only at the city office. Calling Etihad was largely useless as you mentioned, no one seems to know exactly what and where you can use them.
Now, Etihad have an office in Midtown Manhattan where you can visit and book a ticket and use the voucher towards payment of the ticket. But for those people who live far away from New York City, they have no option, the voucher is largely useless as unless they visit the ticket office themselves, it cannot be used online or over the phone so ends up wasted.
I don't know how many other airlines have a similar set-up as to using these vouchers, but for Etihad it's a great advantage as they get to give away vouchers which only a small percentage will end up being used.