Re: OP. This topic comes up quite a bit. The Upgraders point to cost differentials between cabins as being greater than the lowest coach ticket (which they usually are), but of course the real value is what YOU perceive it to be, not the airline. You've also got personal situations where it's enhancing a coach ticket for business, or the person has limited vacation time. Quite a few would apply miles toward some spcial hang-the-moon trip with an S.O. to Paris, Phuket, etc.

I've always been in what I'll call the Plant the Flag camp, i.e. have only spent miles toward free tickets, not upgrades. Though I haven't turned down operational upgrades from back when I had legacy status (and those were nice).

A Plant-the-Flag person might be much more strapped on funds but better situated for time. I think an essential quality to make it work is to have a big bucket list of places. One of my specialties was spending miles on hard-to-reach spots due to monopoly routes or some other reason. I made 5 or 6 trips around Micronesia back in the early and mid-aughts on CO with a fave award as MNL to one spot, like MAJ, with a stopover in another, like PNI, and maybe an overnight in GUM on top of that for 25K miles. Got to all the Micronesia stops for at least 3 days except TRK on various trips (plus ROR, Yap and Marianas).

Did a similar thing in the south layer on NZ using UA miles thanks to the great CO-Amtrak-UA caper unearthed on Flyertalk that let me move 275K miles before they shut it down. The focus there was NZ awards from Australia as far wast as Tahiti, so you could go PER-AKL-PPT (stop)-AKL(stop)-PER at 25K. Same kind of strategy as with Micronesia.

Today the awards cost more in miles and the earning-by-flying is under attack. Also many more regional LCCs to go places. There were a few places I had my eye on hoping to go on miles, like the Seychelles and Madagascar, but it was a paid ticket that finally did it for the former and the latter is still out there. I used 21-year-old KE miles on a key ICN-ULN RT in 2013 to see Naadam in UB. But there are fewer cases like that compared to before.