A two-stage skillchain (created by two Weapon Skills) will result in a 160% increase in magic damage on the magic burst.
All stages beyond that will result in an additional 10% damage increase on magic bursted spells (a three-stage skillchain will yield a 170% damage increase, four-stage will yield a 180% damage increase, etc.).

There you go. You asked and they provided.....a long time ago! Not to mention that a multi-stage skillchain allows for multiple spots throughout the process for a mage to burst. As I stated on the other discussion, in a 3 part SC I can burst 3-4 times, where on a SCH sc I can only burst twice.

And the negatives for SCH sc: A lot less damage. Negligible if the mages are killing it in 1 volley of bursts, sure. There are also plenty of mobs that either negate (hydras), absorb (tons of different mobs), avoid (any mob with shadows) or are impacted in others ways by magic damage. Some mobs rage when you hit them with a certain element. You said its all positive. Well here are some negatives. They can be overcome in certain ways, but so can the negatives for DDs.