Yes, I know getting miles on another carrier is great, but all that could be done on the computer WITHOUT actually assigning multiple flight numbers. However, there are so many downsides to codesharing.

1. Airport arrivals boards, timetables, etc are filled up and made confusing with multiple entries for the same flight.

2. Passengers go to the wrong terminal for their flight, or cannot find a check-in for "their" airline.

3. Passengers have no idea who is actually operating a flight.

4. Different standards of service, eg BA is now codesharing with Swiss, where Swiss charge for on-board meals.

5. Tedious pre-flight announcements about different flight numbers which just sound gobbledygook to the normal traveller.

I have never objected to commuter operators painting their aircraft and flight numbering them as part of a major airline - that is part of the major airline's method of working. But sticking a whole string of extra flight numbers on a flight (Star Alliance are particularly bad at doing this on each other's flights) is just so confusing.