I leave mine in as well. I used a polisher/sealer last summer on the bottom and it worked very well. Whether it slowed me down who knows? But it was a lot easier to get in the water and brush the green stuff off than just leaving the hull untreated.
This products has some endorsements like everything else. Like all products, it has supporters. I doubt it does much against growth or buildup, but if it did impact speed it would be good.