Only time I think this may be " acceptable"
may be when your flight on airline A gets cancelled and you are then rebooked on airline B and ends up getting miles on both, whether intentional or not. Since I normally choose my original flight because of miles or some other economic/covenience reason I have no qualms about getting (or even requesting) miles from A as a minor compensation for the hassle.
Mike
[This message has been edited by MIKESILV (edited 12-02-2002).]