As frankvb says, an MFU, especially from WT+ to CW, but also sometimes from CW to F, is the often the most effecient use of miles, apart from when you can get double-value from a miles redemption as in last week's sale or an Amex 2-4-1. However, there are exceptions to bear in mind.
For example:

• When I flew to TLV a couple of months ago, an MFU was virtually useless as the WTP fare wasn't much less than the CW fare (and this wasn't during a CW sale). However, at 40k miles for a return, a CW full redemption was very good value.
• Miles redemptions are fully flexible and refundable and the typical T-fare is not. The value of this flexibility obviously varies from one pax to another.
• Booking at the last minute the only available fares could be very high yet there could still be redemption seats available. This can even make a short-haul miles booking in Y (or even better, "Part Cash, Part Miles") worthwhile; but will also be true of travelling long-haul in F at just a couple of weeks notice, when the CW fare is likely to be very high.
• Some people just earn so many miles (typically on business) that they simply can't spend them all just on MFUs.