The idea that you can't mix is ludicrous. Even in "matching" systems, the rears tend to be different anyway, in terms of drivers, driver size, sensitivity, impedance, power ratings etc etc.
I've always had different rears in the past, although I recently downgraded my main TV system so now they are all the same brand other than the sub.
Why would a matching system "tend to be different anyway"? If you have identical speakers all round the only thing standing in your way of total cohesion and uniformity of the soundstage would possibly be speaker placement and your room. Now, how easy it is to set up identical speakers might be another issue, depending on what speakers you have and what your room allows.
After years of mixing speakers (models and sizes, not brands) I'm myself now using identical satellites front and rear, and thus experiencing clearly audible benifits which makes it "impossible" for me to go back to anything else. Natually, it depends on how picky and demanding you are of your audio reproduction, and perhaps on how sensitive your ears are.
I get the impression people often dismiss the relevance of identical speakers all round simply because they haven't had enough (or any) true experience of it. The difference can be substantial, and thus the advantage dramatic (compared to an ill-matched speaker system). I look at it like this: as audio is reproduced through my speakers (music or film alike) I dont want the tone to suddenly change, sounding 'off' and different depending on which speaker its coming from. That tends to pull me out of the listening experience, rather than pull me in. Its not without reason some of the finest mixing- and recording studios in the world use identical speakers all round.