With a mag conversion you can adjust the speed of the reel there and then to suit the conditions to get the best distance from your reel. On the downside you loose the ratchet on most conversions and ive herd they can get in the way when reeling in aswell as the reel loosing value.
Personally i dont think you need them on fishing reels, you can easyly control your reel with your thumb if fishing into a strong head wind or side wind, if you want to add to your casting distance improve your casting not your gear.