This is probably the most generous free ticket of its kind, since not only are there no fare restrictions, but you can also use it with an award flight. (rewardexpert.com)
Or who knows, maybe there will be some freak fare to combine this with as I don't off hand see fare restrictions. (travelisfree.com)
While this can be useful, it's worth noting that there are often fare restrictions that require you to buy a more expensive ticket first in order to get the second one at a discount. (travelcodex.com)