The problem with air travel is the speed for the cost. An economy seat is far more justifiable for the amount of time you will be using it compared to other modes of transport. Unless all you do is fly, then really there is no need to waste money on upgrading.
The only time I could ever see it is flying across multiple hemispheres, say from NA to Australia, or SA to Asia. Anything less than or equal to 12 hours is not worth the cost, unless you are a fat fuck. Just fall asleep (unless you can't, then you are just a pussy).