I think it's most likely a "to each his own" situation. From an economics perspective, it's maximizing one's utility. Using your various forms of currency, whether it be dollars or points or miles, etc. in a way that you enjoy the most. Makes plenty of sense to me, even if it's not what I would prefer to do myself.
Personally, I have not flown on a miles ticket since before I first learned about and qualified for frequent flyer status 5 years ago. I have used a few companion tickets on some expensive and/or low upgrade chance routes (around Thxgiving and Christmas, when fares are high; and on ATL-EGE ski trips, which saves a lot of money, and is a route that usually has a full or nearly full F cabin prior to upgrade windows....lots of money going in and out of EGE).
For me, I would rather pay for my tickets to earn the miles and elite qualifying miles. I also engage in some MS in order to get elite miles from our cc's. The goal for me is a higher level of status, so I end up redeeming points for gift cards and hotel rooms, even if it means less than $0.01/mile of "value".