Gary, did you intend that Prismatic Sphere be usable only for defense?
A common tactic (well, any casting of Prismatic Sphere was very uncommon, but anyways) was for the caster to stick his hand through the Sphere and fire spells, with 90% cover as a result (or, if applicable, he could stick his hand out the OTHER side of the Sphere from his opponents, and cast a spell that did not require direct line-of-sight.)
Could, however, a mage simply sit totally within the Prismatic Sphere and cast, with his spells passing freely out of the Sphere? (the most powerful interpretation, of course.)
And ...
(humor)
How come, when the Prismatic Sphere was created, that it did not immediately disintegrate the soil it touched (half the Sphere goes into the ground), thus causing the soil above and within the Sphere to fall into it, and thus eviscerating all the soil the mage was standing on, until he was standing on his own Sphere? (of course, this would have been bad for the mage's fellow party members, who would have fallen INTO the Sphere, and thus suffered all of it's effects, and ...)