I don't plan on defending the idea of that many channels, but if you have normal human hearing, you can hear the difference between a sound coming from in front of you and one coming from behind you, so going with a 5.1 channel system is quite appropriate for humans. And so, presumably, you can understand why that might be desired in movies.
I have several SACDs that are multichannel classical recordings, that when I put them on, I do not consciously hear the rear channels from my normal listening position. But I can certainly hear the difference with them shut off.
In a real auditorium, you hear most of the sound from the front, but you also hear reflected sound from the wall behind you. (You may not notice it in the auditorium, but you would notice it if it could be switched off; it would then sound quite different.) A good multichannel recording can simulate that better than any two channel recording can. However, one must take great care in setting things up properly, or otherwise it will not have the right effect at all.
One can save money on the main speakers by getting high quality bookshelf speakers instead of more full range ones due to the fact that one will be using a subwoofer (or more than one) to deal with the deep bass. (This is, of course, assuming that one is competent to set it up properly; otherwise, one would be better off with a simpler system.) In my case, I am using bookshelf speakers that retail for $1500; the tower speakers that are voice matched to them (with the same tweeter and midbass driver) cost about twice as much, and do not go as deep as my subwoofers, so I am better off this way than I would be with just the pair of tower speakers, even if I am only listening to a 2 channel source.
[In fact, I would want to use a subwoofer or two instead of going with the tower speakers even if I were not using a surround system, as the bass can be better for less money that way, though it does require more care in setting things up, and if one is not up for doing that, then it will not work well for one. But that is a defect in one's ability, not in the concept.]
Now, I do agree that it is not a good idea to go with a cheap surround system instead of a decent 2 channel system. Though I recognize that different people have different standards on this, I would not bother with a surround system on a budget less than $1000, and likely not on one less than $2000. But, of course, one can do a surround system by going a bit cheaper on the rear speakers, as they matter less than the front, but that is less than ideal, and I hope to never have to do that again.
