Yes we all have a different way to view miles. here is how I decide.

I used to keep them to fly the Concorde. Done. Then I kept them to get a loved one to fly it too. Done. No more Concorde now, and I have no other big plan that would cost me much less using miles.

So I do not keep more than about 100 000 miles unused, airlines change their policies too fast. When I have this kind of miles, I treat myself to business or first class, particularly transatlantic, particularly when there are no cheap tickets (Summer, Christmas).

I value miles at 1 cent, and deduct the earned miles from the price of a ticket to calculate the "real" cost. If the ticket is worth less than the amount of miles for an economy award, I generally buy it. If it is worth more, I sometimes treat myself to first or business (particularly because it often means economy will be full).

Finally I think it is still good to have about 50k miles aside, in case of emergency : last minutes tickets are often expensive while miles bargains are often available...