Worth every penny. Compaction can seem fine at first, but it doesn't last on a steep slope. Water always finds a way in. Once it does, the soil washes out fast. Geotextiles act like a net to keep things in place. That extra cost saves you from a huge repair bill later.
Yeah but when you say it acts like a net... how's that really work? If water flows right through the fabric, how does it stop the soil from washing out with that water? Seems like the fine stuff would just slip through the holes...