No good reason NOT to run one, but they're not necessary on carbed setups with stock or street performance level mechanical fuel pumps. They keep fresh, cool fuel circulating through the systems, which is only ever a good thing. They will help alleviate vapor lock and other fuel heat related issues. But you need to size them properly for your setup. They cannot present a restriction to fuel flow. When I switched to a Walbro 255lph fuel injection pump to run with my carb, I was required to add a return line and the appropriate regulator. In my case it needed to be a regulator that can drop the high pressure of the EFI pump down to carb-safe levels. So my supply and return lines are the same size. If you run a return line in this setup and you can't get your fuel pressure low enough, it's a sure fire sign that you have a restriction in your return. On a carb setup with a mechanical pump, you can run a smaller return line since the amount of fuel to return is typically less. So a 3/8" feed line can utilize a 5/16" or even 1/4" return. The pros say not to use a typical dead-head style regulator to run a return line, you want to use one that is specifically designed to run a tank return. I've also heard of guys having success by just adding a tee in the fuel line and running it back to the tank. I don't think I'd do that in any sort of performance application though. Sounds like a recipe for fuel supply issues or cavitation. Anything that prevents the fuel from dead-heading at the regulator is a good thing.