I too was told a ruler is more accurate than the mat when I first took a class. But I use my ruler instead of the matt for the following reasons:
1. I am not constantly cutting on the same line thereby wearing out my matt on an often used measurement like 2" or 2 1/2".
2. I am not limited to cutting in a certain direction or position by using the ruler instead of the mat.
3. Can't imagine cutting a 1/8" measurment using the mat/
4. Rulers are much more portable than mats, especially if you have a big mat. It is best practice to always use the same measuring tool when working on a project so I can cut at my machine or at my cutting table by using my ruler.