A savvy traveller willing to spend a Saturday night away can take real advantage of this and I am surprise the benefits have not been spoken about more here on FT.

For example, if you have a meeting in New York on Monday 11 November and you don't know whether or not you will need to go to dinner with your associates afterwards you have three options:

Inflexible: out Sat 9 Nov, rtn late on Mon 11 Nov GBP 2,503.85
Semi-flexible: out Mon 11 Nov, rtn Mon 11 Nov at your convenience GBP 4,298.85 (with GBP 200 change fee)

outbound inflexible, inbound semi-flexible: out Sat 9 Nov rtn Mon 11 Nov at your convenience GBP 3,401.35, a saving of GBP 897.50 on the semi-flexible fare, or GBP 697.50 if you make a change.

Hotels and subsistence might cost less than GBP 697.50 so it could make good financial sense!