I've always justified reaching Platinum at 50 nights to interested colleagues by calculating that around 10 of those nights will be free on points (which has historically been the case) and I've spent an average of $125 a night on the remaining 40 nights so a total spend of $5000 a year for 50 nights that include tangible cash savings of (2 x breakfast @ $15 each) + (2 x 5 alcoholic drinks @ $5 each) + (2 x evening meal) (maybe 50% of the time) and the intangible benefits like 4pm late checkout and suite upgrades which means the actual room rate comes down quite considerably to *free* if the food is good or certainly less than $50 a night if we just hit the booze for a few hours.
Of course, I'm in Asia so get these benefits at Le Meridiens, Westins, Sheratons, Marriotts, Courtyards, Ws etc. on pretty much every stay. And I've hit 75 nights the last few years with ~75% leisure at these hotels and rates. And I'm now redeeming points on more aspirational hotels like the W Samui which would have cost $750 a night when I was there and seemed a good deal at 60k points so I have fewer free nights.
My average spend is also increasing a bit now because I have some work travel to Europe where room rates can be $350 a night or more and skew the actual results. But when anyone asks about the value of being Platinum I run through the numbers shown above.