After having LSi15 fronts, LSiC center, LSi7 rears, and LSiW sub for about 2 years I upgraded the LSi7s to LSi15s last week. Great move. The sound is much more integrated, and just sounds better overall. The added rear bass can be felt in my chair for more punch during a movie.
I have heard over the years many people comment that the rear speakers are not that important since there is little rear channel energy in a movie. This might be true for some movies, but in other movies the LSi15s are most apparent.
So far, on "Master and Commander", "Fifth Element", and "Terminator 3" they have made a big difference in sound quality. Its almost like watching a new movie.
To me, the moral of the story is that, for optimum home theater, all speakers should be identical.