Having a metal back must have some effect on reception. It MAY be too small to make it go from x bars to y bars and it will possibly have more or less effect depending on which direction the transmitters are in, but lets be realistic here, radio waves will travel much easier through rubber and plastic than they will through any kind of metal.
So the difference in some situations may be very small, and in others may be significant. Personally I would rather have the strange rubbery back that comes with the S5, knowing 1: I am guaranteed the best possible reception at all times, and 2: because of what it is made of, I cannot easily dent, scratch, or damage the back and 3: It's construction will allow for better grip than a hard metal back so I am less likely to drop it.
Samsung themselves have said the reason for lack of metal back is due to the effect it has on reception. And this is what worries me, if they get pushed by idiots and the media into putting metal cases all over it for no good reason, we lose points 1-3 AND inevitably have to pay more for the privalege. No problem if people want to buy a 3rd party or even Samsung original metal back but I hope to god they don't start shipping them as standard.