You also need to take into account your capacity for travel. If I only have time and interest in two vacations per year and don't need air travel otherwise, that sets an upper bound on the travel valuation I can count.

I can use the value of the tickets for those trips, but if I earn enough miles every year for a 3rd or 4th trip, I am not going to use those miles for airfare, so I can't value those miles as if I was redeeming them for tickets. In almost every case, the next best alternative after paying for your travel (airfare and possibly hotel) is cash. 2 Cents back per dollar on a cash back card is better than you're going to do trying to extract value from airlines miles for anything other than airfare, so I think you always get cash back after you've earned enough miles to cover your trips.