Expanding bit by bit is extremely fragile, arcanely complex and error prone. Not worth the amount of financial savings, especially when the cost of time and effort is added on top of that.
It might be what you say, but in many cases it might also be a necessity. I built my first array with 10x1TB disks (starting with 3) for my home server when I was a student/got my first job and could only afford one disk every now and then. Linux' expandable mdraid worked like a charm multiple times. I probably wouldn't have bothered (and would have lost data or limited my hoarding) if I had to save for a year just for a fileserver. I am not talking about precious data but data don't have to be irreplaceable to want to protect them.
