It's not all or nothing!

You should use the FF program when it doesn't cost you anything significantly more, and (especially if you're not looking to reach a plateau for an elite level) you shouldn't use the FF program as your only factor.

Furthermore, at least in the US, if ALL you're redeeming for is coach flights that would be cheap anyway, on the creidt card side, it often works out better to simply get a "cash back" credit card. The REAL value in FF miles is if you redeem (and CAN redeem*) those miles for longhaul international upgrades and/or longhaul international flights in business or first class (in situations where sitting in that higher class is really worth it to you). Those flights are SO expensive that you might consider them "unaffordable" for ordinary purchase. If you are a tall person like me and sitting in business vs coach makes the difference of whether you'll take a 10+ hour flight (for leisure) or not, then it's definitely worth SOMETHING extra (even if "unaffordable" normally) for those classes.

But if you all you're redeeming for is "ordinary" flights, you have to look at what it costs to redeem when you can redeem (it's probably only when there are really cheap sale tickets available anyway!), and work that out into a rough cost per mile for you. Then when you can choose between a price of something (flight, hotel, anything) that earns you miles vs one that doesn't, work out the math of whether you'll earn more value in the miles than you'd save in the cash.

*How hard it is to redeem depends on a LOT of factors. First, it varies by airline FF program (not just by alliance!); for example, QF is known to be a LOT stingier than OW partner AA. Second, you have to be flexible and plan ahead: Availability is often best either about 11 or about 3 months out (but that's just a very rough statistic, it's no hard and fast rule).

Also, it CAN reduce to simple the value of miles: Here in the US at least, most airlines let you get an award ticket with HARDLY ANY restrictions as long as you use double the miles (they each have some name for it, like "saver" awards are 25k for domestic coach but "standard" awards are 50k). So with those program when people say the can't find awards, what they really mean is they can't find discounted awards (just like they might not be able to find discounted cash tickets!).