(apologies for the format, this pesky website keeps unremembering that I'm logged in)
David F quote
Surround channels, contrary to popular belief, are important. All channels are important. The centre is important as it reproduces virtually everything that happens on screen, and has to produce clear dialogue at the same time. The left and right are normally producing music, but are also producing left/right effects - these have to blend perfectly with the centre (or vice versa). The subwoofer is important as it underpins everything, bringing weight and solidity to big effects. Whilst surround speakers may not be producing dialogue, they are producing ambient information, and they have to blend perfectly with the front to draw the front soundstage out from being a single flat dimension. Depending on the mix, they sometimes produce music as in the film Se7en - try being creeped out by the music when you’re using some tiny little speakers that are lesser quality than the fronts. Skimp on the rears at your peril!
[end quote]
Couldn't agree with this more. I had a little experiment recently. Currently I have three well-matched speakers at the front (floorstander, matching centre, floorstander). But I had been using some Monitor Audio Apex A10's (just satellites, albeit at the higher end of what you'd expect from satellite speakers). Anyway I swapped them over with some Ruark Epilogue 2 bookshelf speakers I use in my other system and played Jurassic World through Apexes, then the Epilogues. (Yes it's not the most intellectually challenging film around, but the surround mix is very well produced and very effective)
The difference was remarkable. Obviously the timbre of the Epilogue's drivers match it's same-branded front three more precisely, although the Apexes weren't overly different, but the main difference was the frequency response. The Epilogues could go so much fuller in the midrange before crossing over to the sub. There was one scene in particular when they were attempting to locate the Indominus Rex in its enclosure (I mean it must be really tough finding a ginormous dinosaur in a pen!!), and they hadn't realised that it was behind them and it roared though my rear speakers. Through the Ruarks it honestly gave me a real jump! The depth of the roar and sheer presence behind me was quite a step above the diminutive Apexes, as good as they are at what they're designed for.
But like DavidF said, it's not just effects, it's the sense of soundstage, which tends to show through on more dialogue-heavy films where you can just sense more accurately the size and shape of the room the particular scene is taking place in.
Anyway, that's kind of a long-winded way of saying yes, the rears are important. Although tempering that, I would be careful about overdoing the levels you set on the AV receiver for the rears. They shouldn't be overwhelming the mix as a whole, just contributing to it.