What is the worst possible thing to get with miles?

Not sure if it's the worst, but using your miles to partially pay for your flight is one of the worst ways to use your miles.
Note there is a difference between using your miles to pay for flights, and using your miles to redeem flights. The latter is what the miles game is about, while the former is a waste of miles.

The worst case of wasting miles would be to let them expire. I've seen some of my friends travel for work, and then forgot to redeem their miles, losing almost 100K in the process. Another friend panicked and redeem economy, better than letting the miles expire I suppose, but again, not the best use for miles.

Miles are best used for J and F, and that's how I'd use them. To avoid this, you would want to ensure you either earn miles that don't expire (some cards have this feature), or earn enough miles such that you can regularly burn them with J/F redemptions.

Still, as the rest have mentioned, using miles for Krispay, Tap For more, etc, is an option if your miles balance is really really small and you just want to squeeze a little more out of it instead of letting it expire totally.

Letting your miles expire is definitely a concern for miles that have a shelf life to them. Not just losing the miles, but also the opportunity cost of you forgoing the cashback to accumulate these miles.

The following is a personal choice as well, but I do know of some who redeem the miles on budget airlines such as Scoot. It wouldn't say it's bad/worst as it is a choice that they make from the get-go (which I believe they have already done their prior cost-benefit analysis).

My point is that it is always good to begin the miles journey with an end goal in mind in terms of the redemption you are aiming for. For example, accumulating 80k miles for a round trip to EU/US.

For me, those messages banks send to USE YOUR MILES TO PAY BILLS - I see people clicking yes, and as much as I hate it - it does get to some people.

Hence, I would say dont use your miles to offset bills :)