We've beaten this topic to death here over the years. The main thing is to have a framework for the value of a mile. As someone who is constantly manufacturing miles rather than getting them from flying- I just want to make sure I'm not redeeming for lower cpm than I'm purchasing them for. I tend to hold out for 2cpm or more, although I do make exceptions for orphaned amounts of miles that I won't ever have enough for a high-value redemption.
If you're the kind of person that mostly earns from flying on other people's money- it's probably OK to just consider the miles as free tickets you found in the sofa. At the end of the day it's an issue of whether you see miles as scarce or plentiful.