Are there any scenarios whereby it's worth it to redeem economy/ premium economy with miles?

I would say 2 scenarios for me

1. Miles are expiring anyways

2. I dont have enough for business and I know I will not have enought by then

The only scenario i can think of that its worth to redeem economy class:-

• you have miles that are expiring and you better use it before they are gone

• and that is either you have only enough miles to use the economy, (or cash + miles for economy).

• Or business class is full, and you can't wait any more because your miles are expiring​​​

I redeemed an economy ticket once, a week before christmas. The revenue price was almost $1k and Business Class was on waitlist. At $1K revenue price, redeeming for economy suddenly became worth it.

Redeeming miles for economy class will make sense if the ticketed prices are high, like holiday seasons or sectors like SIN - PVG for example.

If you want to use less miles, wait for SQ Spontaneous Escapes for a (usually 30%) discount. Take note that tickets for this promo is usually for travel in the next month after (with exception of USA flights which may be 2 months ahead at times).