Well, if we look at this a little differently - if you had unlimited money, you wouldn't stop at 2.1, right?
So this isn't about the experience of the sound all by itself. It's a value proposition. Better to get 5 affordable speakers and a sub, or two, more expensive speakers and a sub for the same total price?
And to me, that all depends on what sort of price point we're talking about. Better to get a 5.1 package that includes $2000/Pair tower fronts, a $1000 Center, $1000/Pair surrounds and a $2000 subwoofer; or, $4000/Pair fronts and a $2000 sub? Not such an easy decision given how many really good $2000/Pair tower fronts are out there right now!
But if we're talking about a $250/Pair set of tower fronts and a $350 sub vs. an all-in-one $600 5.1 package, now we've got a completely different story! I'd be telling anyone in that situation to get the $250/Pair towers and the $350 sub, then save up for the matching $125 center and $125/Pair surrounds!
But I think you're asking if you should just stop at the 2.1 system and not even bother with the idea of saving up for the center and surrounds. For me, I could never do that. Although I would sooner go without a center speaker than I would without surround speakers. Great front Left & Right speakers can create a very convincing "phantom" center. But if I had a large, wide seating area, I'd want that center back in a jiffy
I'm ALL for starting with a 2.1 system and saving up to buy the surrounds and center later. But I can't say I'm for stopping at the 2.1 setup and never giving the other three speakers a second thought. I do NOT believe it's necessary to go nuts on the surround speakers though. Like you say, they're really just about ambience, envelopment and the occassional sound effect. If you're going to skimp anywhere, skimp on the surround speakers! But I wouldn't excise them entirely. I'd just be willing to spend a lot less on them than the Front three
