Well, they are practically identical to other range touch energy spells with no save except:
1) They have no Spell Resistance. This is big at high levels. The main defense against Orbs at high level is having cover/concealment (or the caster rolling a one which is unlikely) or having mega-hit points and the latter will not last long. Even energy resistance is somewhat limited against them since some casters like Warmages can pick and choose which to cast.
2) Most of them have an additional one round effect (or in the case of Orb of Force, it does a lot more damage than other comparable Force spells instead and has few defenses and can blow through incorporeals). "Oh, I dazed you? Great. I get to Orb you again next round and there is nothing you can do about it.". Not exactly the same as save or die, but close enough.
3) They blow through Golems.
4) If interpreted one way, they blow through Antimagic Fields.
5) If interpreted one way, they stick around forever. "Hey Johnny, come play with this Orb of Fire".
6) They are Conjuration energy spells that are stronger than any similar Evocation energy spells. That's just plain wrong.
7) At high level, they often do 50+ points of damage which results in a save versus massive damage. Even if the save is easy to make, anyone can roll a one and there are even high level opponents who might have a 25% or more chance of missing a DC 15 Fort save. It's not always 5%.
8) Like other range touch spells, they do double damage on a successful critical. And, sneak attack damage adds to them. Although this is true for other range touch spells and not unique to Orbs, the fact that there are few defenses against them means that about 4% of the time, they are nearly an insta-kill spell (shy of extenuating circumstances and in addition to the massive damage chance). The other range touch spells are not quite this bad due to fewer defenses (and lower damage caps).
9) They do what no other 4th level (or higher) spell does: extend a single target energy attack to 4th level which raises the damage cap to 15D6. One set of balance to high level area effect energy spells are that a) they have saves for half damage, b) they have spell resistance, c) they have situations in which casting them is harmful to the party. This spell does not have these limitations and introduces a damage power level beyond what normally can consistently be done at higher level.
10) Orbs are also a great mop up spell since they tend to do average damage most every time and there are few defenses against them. The Fighter wounded the BBEG? Great. Mop him up with an Orb. Round one or two, the BBEG is dead. Next.
11) Metamagic to improve damage? Orbs are low enough level that Empower and Maximize can both be placed on them even without Metamagic Rods or Sudden Metamagic feats or Arcane Thesis or Incantatrix metamagic abilities.
People might not be bothered when their PCs throw these around, but just let a bunch of NPCs start chucking them at the PCs and see if your players think that are ok then.
