Two words: "Yeah, right."
I've played enough 3e to know that, past level 10, the majority of foes you face that isn't an NPC will have energy resistance or immunity of some kind. Just look through the Monster Manual.
CR 10s
Animated Object (collossal) sort of--hardness
bebelith--surprisingly, no
couatl--no
Formian, Mymarch--yes
Fire Giant--yes
Clay Golem--no but magic immunity
hydra, 11-headed--no
hydra, 9 headed pyro or cryo--yes
Naga, guardian--no
Rakshasa--no (not that it matters with SR 27)
Noble Salamander--yes
Slaad, gray--yes
CR 11
Barbed Devil--yes
Cauchemar--yes
devourer--no
elemental elder--varies (water and fire--yes)
cloud giant--no
Stone Golem--no but magic immunity
Half Celestial Paladin 9--yes
Half-Dragon Celestial Lamasu--yes
Harpy Archer--no
Hill Giant Wereboar--no
Hezrou--yes
Hydra--varies (cryo and pyro)
collossal monstrous spider--no
retriever--yes
troll hunter--no
dread wraith--no (though 50% immunity applies)
That's actually a surprisingly small number (about 50%) based on my experience. However, nearly all templates add some kind of elemental resistance and templated lower CR monsters (who may or may not have had resistances to start with) account for a lot of high CR challenges which may explain the difference between my experience and the Monster Manual.
At higher CRs, pretty much everything has energy resistance of some kind (there are only 9 or 10 monsters without energy resistance past CR 11 in the monster manual).
If the writers of the Planar Handbook assume that energy resistance is not a frequent occurence in a regular campaign, they have a really whacked out idea of what frequent means. . . or they use only a very limited selection of high CR creatures fromt the Monster Manual and don't use templates to create high CR creatures from lower CR ones.
Note also that the ability to select an energy type is, in many ways more advantageous than the ability to substitute half of a spell's damage for force. Selecting energy types freely enables you to target a creature's vulnerability (and a fair number of those foes DO have a vulnarability). Also, selecting energy types will usually enable you to bypass energy resistance entirely by targetting devils with lightning and demons with sonic energies. Substituting half of a spell's damage for force enables a reduced effect against incorporeal and energy immune creatures but isn't nearly as advantageous against creatures with resistance. For example, substituting half of a chain lightning spell's damage (let's say 14d6--average 49 points on a failed save, 24 points on a successful save) for force against a creature with electricity resist: 20 results in 29 points of damage on a failed save and 12 points of damage on a successful save. Without the force substitution, that's still 29 points of damage on a failed save and 4 points of damage on a successful save. Against a creature with electricity resistance: 10, it's 39 vs 39 points of damage on a failed save and 14 vs 14 points of damage on a failed save. It's useful for spells that deal their damage in small increments like the secondary arcs of a chain lightning spell or scorching ray but not significant for big boom spells like meteor swarm, delayed blast fireballs, and empowered cones of cold except on a successful save--and maybe not even then.
Psion said:
As a side note, the presumption that has been made by some in the course of this debate is that the ability to pick your energy is a telling advantage for psions, in part based upon the presumption you will regularly run into energy resisting creatures.
The planar handbook explicitly offers sorcerers an ability to replace half the energy of their spells with force damage. This says to me that the presumption is that energy resistance is not considered to be a frequent occurance in a typical campaign, but in a planar campaign where outsiders and extraplanar template creatures are more common, it is considered telling.