For me, the big plus is cutting patches & strips for piecing accurately. The price of dies is high, but I will use a triangle die for dozens of quilts and cut hundreds, if not thousands of accurate pieces. A curved patch (apple core, winding ways, drunkard's path etc.) is a snap to cut and will actually fit when you piece it. Again, these are dies that I will use for lots of quilts, so if I average the cost of the die over the times I will use it, the cost/quilt really comes down. I'm not a good cutter with just a ruler so an added benefit is I rip a lot less if I start with something that's cut correctly.
If you are thinking about getting applique shapes, you really have to look at how many cuts per shape you are going to do to decide if it's going to pay off. If you were going to do an applique vine border, cutting a hundred leaf shapes would merit buying a die, but cutting 10 flowers may not, although you can get really good sales on dies.