I am a strong advocate for full-service carriers but one of the frustrations that I face with heavily discounted Economy tickets is the low mileage accrual rate (yes, I am looking at you, Singapore Airlines!). Airlines regularly run promotions with exceptional rates - for instance, you can get a return trip to Kuala Lumpur and Bangkok for just S$158 and S$188 per person respectively. These promotions generally require a minimum of 2 (or 3!) to travel and they are more likely than not in a fare class that earns you only 10% of the actual miles flown.
You should always check the fare class (especially if you are flying in Economy) to find out how many miles are you getting exactly - you can then decide whether flying on a discounted ticket is worth it since there are usually blackout dates and travel conditions that you will have to fulfill.
Flying from SIN to BKK is approximately 875 miles (you can always estimate the flight distance with an online tool like the Great Circle Mapper). Airlines will usually display their fare class one way or another - all you have to do is to click on that little "i" icon under the fare and it will show you what percentage of the actual miles flown can be earned from this flight.
For more information, scroll down to the bottom of the page and click on the "Full fare rules and conditions" link - this will allow you to see which fare class the ticket is for exactly. Knowing which fare class is important since it will allow you to know how many miles will you roughly earn for the flight - remember these miles are usually elite-qualifying miles (find out more achieving elite status with the KrisFlyer program here!).
Unfortunately, heavily discounted tickets (especially on Singapore Airlines) generally mean lower mile accrual - you will only earn 10% of the actual distance flown on this flight. Since the distance between SIN and BKK is around 875 miles, you will probably earn only 88 miles on this leg of the flight (using a right credit card for lunch can easily earn you 100-200 miles instead).
With all that being said, I recently chanced upon Where To Credit which essentially tells you where you should credit your miles in. While in this case, flying on a 'K' Fare Class does not earn you anymore miles in other programs, there are various instances where you are better off depositing your miles somewhere else.