I suspect age has something to do with this. A huge armchair probably isn't going to be very good for your neck and back for very long periods; the way you'd have to hold your head for working at a desktop computer (or, god forbid, looking at a laptop) in such a chair would be horrid. If you're young, you won't notice. If you're 30-something, it won't take long for you to know why it's a bad idea.
Good office chairs, Aeron included, encourage (though none can enforce) reasonably good posture. It's not a cure all, but I learned the hard way that "comfortable" chairs are not comfortable in the long run if they don't support reasonably good posture.
If you are already doing something stupid to cause back problems, like using a laptop instead of a desktop with a monitor at appropriate height, then sitting in a big ol' comfy armchair isn't going to seem problematic, no matter what.
But, no chair can make you have good practices. Moving frequently, adjusting heights appropriately, etc.