I didn't really get this either at first when hearing so much mention of it in discussions, but Billy Sarokin reminded me on RAMPS that there are times when the two cameras are covering completely different action/dialogue simultaneously and the mix for each camera's shot is not the same. In this case, sometimes you need a seperate mix for each shot, and both of those should go to dailies. Hence the "two track dailies mix".
Another reason to work hard to make a dedicated mix is that post is not necessarily prepared nor budgeted with enough time to remix all of the production tracks from scratch with your iso feeds.  In an ideal world, the post team could take advantage of all of the tools and the ability to automate and go back and tweak and fix a mix, and sit around massaging the dialogue mix to perfection before even going to the dubbing stage. In the real world, these folks are often under the same kind of ridiculous time pressure that we are under on set (or worse). They are dealing with producers who have either completely underbudgeted the sound edit, are running out of money by that late stage of the filmmaking process, have an inflexible deadline to deliver a finished product and meet a pre-agreed upon release date, or any combination of the above. Though the potential they possess for control (dealing with mixing elements of a recording, as opposed to a live occurrence, where one can stop, go back, fix things, etc) is superior to ours, there is often no resource of time to realize this potential. So, aside of your very wise comments about a mix that doesn't work on set potentially not working in post either, we also can help out our brothers and sisters in post by getting it right the first time. It may also help the overall movie and create more time for creative sound design, rather than dialogue repair. And if that's not possible, or we screw it up and miss a cue or whatever, THEN that technology is in place for them to repair our mistakes or inabilities on set by delivering the isos.