They allow you to get more than your .1 gain in each attempt (I think max is .5). The alacrity scroll's countdown timer is logged in time, not real elapsed time. If you do not log the character in except for when it is time to get your gains, then that 15 min duration for the scroll lasts a looooong time for training purposes. The drawback of course is that once you eat the scroll, you won't be using that char until the scroll's goodness is used up.
-Skylark
P.S. Make sure you always log out in a house you are friended to or an inn so you "insta log" and if you are working combat gains, make sure you aren't flagged otherwise you will have the timer to wait out, and waste a big chunk of your scroll's active life.
