Reason people use a float with a jig (from what I've read over the years):
1) Float adds casting distance.
2) Float keeps bait at constant depth.
Regarding #1, I think using lighter gear (particularly lighter line) is a better way to get more casting distance. I use UL rods and 2-3# line and get plenty of casting distance.
Regarding #2, I think just reeling at a constant rate of speed does a great job of keeping the jig at the same depth.
Reasons to NOT use a float with a jig:
1) Most floats sold in the US are terrible at indicating bites. I threw my American floats in the trash ages ago and have been using the Euro floats (with bait) ever since.
2) When you use a float, you are intentionally putting slack in your line. That's almost never a good idea. Among other things, it makes setting the hook more difficult.
3) People most often use a float and jig in shallow water. The float can make a large splash when it hits the water and can scare fish.
4) The float keeps the bait at a constant depth. Without a float, I can vary the depth on the same retrieve. I can stop reeling and let the jig fall. I can lift the rod tip and bring the jig higher in the water column. You can't do that if you're using a float. You're stuck at the same depth.
5) Have you ever seen a bass pro fish a float with a jig or a spinnerbait? No, they just reel constantly to keep the bait at the same depth. They're fishing a moving bait for predatory fish -- just like us. Why should we fish differently?
6) I want as little “stuff” on my line as possible. A simple jig tied to the end of line works amazingly well.

