1) What exactly is the idea behind making ball spells more effective than bolts against a single target (+20 on center)?
This is actually a carry over from HARP's roots (i.e. its cousin, rolemaster), which goes to show that nobody is perfect (i.e. me).
I like the re-interpretation where the center gets full OB and eerything else -20. Nice suggestion RandalThor, have an idea point.
2) OB on ball spells in general - why?
Because it is an attack spell, and it is used against a character's DB, which can also increase. This way the caster can become more proficient with it as he gainst skill with the spell.
On Shields....Nick is correct that there is nothing in the rules that says that a shield's DB cannot be used against Ball Attacks.
In the thread that jurasketu pointed out, I point out what the intention was, and my reasoning behind it.
However, since that never made it into the printed rules, it is up to the GM to decide, based on his intrpretation of how a Ball Attack works.
For those who treat it as an exploding wave front, including a shield's DB would be natural.
For those who treated it as a force bubble filling up from the center point, with the elemental material swirling about like water filling a room, then not including a shield's DB is natural.
Neither interpretation is incorrect or wrong.