Firefly clips are more expensive to buy separately and run. They require additional batteries to run (for the"light up system") and normal elite darts are cheaper to buy and easier to obtain. Also the firefly clip is a little bulkier than a normal 18 dart elite clip reducing mobility ever slightly.
Yes you can use normal clips. The firefly tech in either rayven is just in the clip. You could have glow-in-the-dark fire from the rapidstrike or rampage or any other clipsystem gin, as long as you use that clip and the correct darts.