What AP & Todd said. Species of fish, style of fishing and water clarity do play a factor. Tuna & Sailfish for example have huge eyes and crazy good eyesight as a result, so fluro is really needed for that type of fishery. Whereas Salmon and Steelhead are not generally line shy (or as much) and are often found in colored up water. We've run yellow braid straight to plugs and they just don't care. Really skinny, clear water may require Fluro or a thinner leader.

I like 40# braid for spinners and jigs just for the fun factor and the instant feel & contact with my offering. I can cast it a mile, hook sets are a blast and I can horse fish around. The 3 - 4' top shot is mostly for shock absorption and for a break off point when needed. If I didn't ever plan on snagging up and breaking off, then I'd just fish spinners with straight braid. Conversely, I'd only free drift with mono as I know I'm going to break off...a lot. Losing a spool's worth of braid is hard on other fisherman and the wallet.