Cheaper, yes. More effective... no. I've seen this reasoning and this course of action a lot of times. Almost never people reached the plant mass actually needed. That's why I put such an emphasis on it. It simply never works out. Hope you are one of the few cases where it does. But excuse me having doubts.The other reason is propagation. As you mentioned it's best to heavily plant a tank when first set up. There are a few ways to accomplish this - one is to buy a lot of plants. The other, cheaper method is to buy the plant species you want ahead of time and propagate them. This is what I'm doing with my stem plants