Pyrex said:
The problem isn't so much that such a spell can't be intrinsically balanced, but rather that the designer can't necessarily take into account all the various metamagic feats and class abilities that might make said spell broken.
Actually, I tend to disagree with this.
There are certain "norms" that most spells follow and when designing feats, the designers should take those norms plus the synergy of other feats into account.
In the case of the Orb spells, the designers ignored those norms and designed a spell that was better than Scorching Ray for example.
Until the Orb spells came out, Scorching Ray was the de facto standard for most effective ranged touch attack energy spells. Because of metamagic, there have been discussions on whether Scorching Ray was two powerful. Suddenly, Scorching Ray became semi-obsolete because Orbs had several things that Scorching Ray did not have:
Pro:
1) They took away the SR (which allows the Orbs to affect Golems and other SR creatures with impunity).
2) They added a secondary one round effect with a saving throw.
3) They increased the damage cap.
4) They changed the school of magic such that one interpretation of the Orbs allows them to pass through Antimagic Field.
Con:
5) They had the Orbs fire one more powerful missile instead of three lesser ones.
6) Orbs are 4th level spells instead of 2nd level spells.
In the Evocation school, the main advantage higher level spells tend to get is the increase in damage cap. Nothing else. For example, Chain Lightning gains increased damage cap and selective target at the cost of less damage for secondary targets when compared to Fireball.
Con #5 here is not much when compared to Pros #1, #2, and #4.
So, I think the designers ignored the "norms" in this case and totally foobared and that is why the Orbs are not balanced, not because of metamagic feats. IMO.