I suppose the major determining factor is how you use your miles and how many you generate in a year. Top-tier fliers can easily generate 300K-plus miles.

An International First Class award on Star Alliance is 120K miles and fares probably average $12,000. That means each mile is worth ten cents.

Of course, I can spend 25K miles on a Saver Award that runs me $250 (or even less). That makes each mile worth one cent.

I can use 15,000 miles to upgrade that $250 W fare to a First Class seat that costs $2500, bringing me back to ten cents a mile.

So using miles to purchase Award Tickets in Coach is usually not worth it, unless you are flying some oddball routing. But to purchase International First Class or Business Class travel, or to upgrade domestic travel to First Class, they can have tangible monetary value.