The problem is that depending on the airline, the cheapest economy tickets can't be upgraded using miles. Singapore Airlines require a full priced economy ticket be used for upgrade and the number of miles required isn't much lower than a business class redemption.
The best value per mile is in long haul business class on leisure routes, where the airline is quite happy for you to sit in a seat at the front which would otherwise be empty. Attractive redemption rates are usually offered to encourage you to burn up your miles. They are much less keen on redemptions on prime routes where you would be displacing a premium fare payer.