How does that help
* people who travel at short notice or on busy flights
* people traveling for business who want to be home at the weekends

Often cheap tickets have Saturday night stay restrictions, or at least 3 day restritctions.

Short Notice
Say you need to pop over to Singapore for a couple of meetings on Wednesday and Thursday next week. You jump on the BA11 Monday night, which has availability in J, on a J1RT ticket. Your return on Thursday night means waitlisting (only available in YWJF) as BA is fully sold out, or you need to route via KUL.

Home at weekends
Say you want to fly to New York in Premium Economy Or Better, departing end of February (so plenty of time), leaving on the Monday, arriving int the afternoon, back to London on the Friday morning, and go direct. Plenty of flights on this route, but the cheapest you'll get is a fully-flexible WTP ticket (W1UKJB) for £2460 - across all carriers.

If you're happy with Economy you can get a ticket in the second highest "B" category on BA (£1132 return, £1052 on Virgin), although if you go out Monday back Tuesday night you're back in Y at £1689.

There are hoops you can jump through - flying half way round Europe to start with, bundling car hire or a hotel, etc. I can't believe that people do this in general though, and I can't believe that a lot of people aren't flying short notice, or avoiding Saturday night stays.