Finally: Do you want to pay for it? Since ambient effects typically aren’t that loud, your side speakers can be small, (just like the rear speakers), but you still need to purchase them, and, depending on your situation, have them properly installed. You also need a home theater receiver (or for your PC, a sound card) that is 7.1-capable. This isn’t an add-on you buy, but a replacement of your current equipment. Home theater receivers with 7.1 surround can cost several hundred dollars, or more.
The answer is probably this: If your current home theater is big enough that you’re sure you will notice the difference, you probably won’t notice the cost of upgrading. But for the rest of us, we may wait until we replace our equipment for other reasons, or at least until 7.1 surround becomes more ubiquitous, or at least less expensive.