Like most miles that you can accrue on an airline, KrisFlyer Miles are best used for Business Class flights and above - it allows you to travel on an aspiration-class of service that you may not necessarily want to pay for. For example, flying on Suites is extremely attainable with careful planning and spending (charging your expenses carefully is not the same as being frugal - it is about using the right credit card!).
While I generally do not advocate using your value KrisFlyer Miles for anything less than a Business Class flight (hey even a Business Class flight from Singapore to Bali is better than an Economy Class one from Singapore to London!), there are some instances where redeeming for an Economy Class award ticket is reasonable.
1. Monopolised or Oligopolised Routes
There are plenty of flights that depart and arrive into Singapore each day and usually, travellers here will have plenty of airlines and flights to choose from. That being said, there are a number of destinations that are only served by a handful of flights (and thus, airlines) each week. Koh Samui is one such example - there are only two airlines that fly directly from Singapore Changi Airport (SIN) to Samui International Airport (USM).
One of them is SilkAir, and the other one is Bangkok Airways. The flight is slightly over 600 miles (similar distance to Phuket) but flight actually costs around S$700 for a return-trip on an 'Economy Saver' ticket. While there are plenty of nice hotels and resorts in Koh Samui, paying over S$700 for a 2-hour flight in Economy Class is not exactly the smartest way to spend your money.
If you choose to redeem for this same flight using KrisFlyer Miles, it will cost you 25,000 miles and S$67.10 in taxes. Therefore, you are essentially using 25,000 KrisFlyer Miles to offset the S$665 fare - this gives you a valuation of 3.3 cents per mile and since you can actually acquire miles at 2.0 cents typically, you are definitely getting a bit of value here. With that being said, remember it is not uncommon to get up to 7 cents per mile on Business and First Class flights!
2. Travel During Peak Periods
Most people avoid travel during peak periods and if you are constantly on a lookout to snag up the cheapest promotional fares on Singapore Airlines or SilkAir, you will probably realise that blackout dates apply for peak periods (e.g. New Year's Eve, Christmas Day or even National Day!). One of the greatest benefits with award ticket redemption is the availability of flights when you plan your travels far out. I love going to Bali and I realise that if I were to take a Saturday morning flight out of Singapore (return on Monday night), it is actually extremely expensive - especially if it is a long weekend. Therefore, using my KrisFlyer Miles for these flights make sense if I have to travel during these periods.
3. Uncertain Travel Plans and Required Flexibility
Not a lot of people know this unfortunately but it is actually possible to change the date of a Singapore Airlines or SilkAir flight for free if you have made a redemption award booking. Say you wish to travel over the National Day long weekend next year but you have not fully committed yet, you can simply make a booking now and then shift it earlier or later next year when required (assuming the miles are valid during the period of travel). Alternatively, if you wish to cancel your booking and completely redeposit your miles, it will only cost you USD 30 (USD 15 for Elite members).