The advantage of bike specific gear is that they are designed for active cycling with appropriate knee room for the movement of your legs, rain pants that are tapered around the calf and ankle to keep your pant legs out of the chain, rain or cool weather coats with longer sleeves for when you're stretched out, rain coat with a tail to keep your butt dry, reflective elements for dusk/night riding and they often take into account that you're generating heat while exercising (zippered vents). Some of these features are available only in the more expensive brands but not always. Regular rain jackets don't have venting because walking or standing at a bus stop doesn't generate much body heat.

Evaluate the weaknesses of what you're wearing, see if cycling-specific gear accommodates those weaknesses and then evaluate whether or not you wish to pay the extra for an item that you may only wear while cycling.