Is 3D printing suitable for repairing a refrigerator shelf glide rail?
Can 3D printing be used for this task to make the repair any easier or more successful to complete than simply cutting a piece of plastic and (again, somehow) affixing it to the shelf?

Yes. 3D printed parts can be plenty strong enough to handle the kind of load you're considering. You'll need to create a 3D model of the part you want it a CAD program, which is something that takes some practice if you don't already know how to do it, so it may or may not be easier for you than milling the from a plastic blank.
One advantage that 3D printing brings with it is the ability to iterate rapidly on your design: you can model and print the part that you think will work, try it out, adjust your model to incorporate what you learn from the first try, and print new versions until you get it just right. If the existing shelf is broken in some irregular way, for example, you can design a new part that mates tightly with the broken edge of the part you have.

The rail was part of a large plastic shelf, which is no longer available for purchase.

Perhaps you've already looked, but there are a great many parts suppliers online. If you have the part number, be sure to look beyond just what the manufacturer can supply -- it's very likely that someone out there has the part you need.