Its actually not only 15%. after doing a custom game, all the spells are 35%... even null zone. and if you play right you have have RoA by 14 minuets, and to myself i like the little extra health since you are a squishy caster that likes to get focused and the more time you can get beat on, the more time you team gets to beat on them.

Personally I find that the RoA is not really worth the time it takes, and does not provide enough AP in the end. For the scepter I personally don't use it, as by the time you get it 500 more health does not make a huge difference if you are facing more than one champion, and 1vs1 by that point you are pretty strong if built properly. If you like the slow go for it, but the 15% is not that much, though with malz's abilities you could get almost everyone on the other team in one round of abilities.
Personally I prefer AP quints, as they help out na lot more in the end than he health.