When they split up the Emotion spell in 3.5, they made some of its former functions into lower-level spells than Emotion itself was (it was only 4th-level in the first place because it had a variety of different uses). Polar Ray really doesn't deserve to be 8th-level, and the 25-die damage cap isn't much use in a core-rules spell.
It was core in 3.5, so using non-core supplements for Twin Spell and Sudden Maximize and Energy Substitution and suchlike isn't much of an argument for why Polar Ray was 8th-level in the 3.5 PHB. Disintegrate can do 40d6 damage anyway (by 20th-level) if the target doesn't make their save, doesn't rely on a specific energy type that can be resisted, and still deals 5d6 damage on a successful Fortitude save. That will generally kill a Rogue, a Wizard, or similar fellow (like a Fey, Undead, Construct, or Aberration, who also have low base Fortitude) outright.
A 16th-level fighter might have touch AC of 18, with a Ring +5 and a 16 Dex in mithral full plate. A 16th-level rogue might have touch AC of 24, with a Ring +5 and 28 Dex in Gloves +6. A 16th-level wizard will have a ranged attack roll of +13 maybe, with +8 BAB, +4 Dex, and +1 Weapon Focus (Rays). Sure, he can Quicken True Strikes, but he still has to deal with energy resistances for Polar Ray. Any reasonably old red dragon is probably going to have Protection from Energy (Cold) active anyway, while white dragons won't care, demons and devils will often resist a nice chunk of the damage, etc. The Polar Ray will certainly be useful against fighter-type NPCs or others who might not have invested in energy resistance.
Evasion is not that common. And Disintegrate works better than Polar Ray for many purposes. Anything it doesn't work well against (high Fortitude saves) will be the stuff Delayed Blast Fireball works well against, or Irresistable Dance (followed by DBF or similar), or Mage's Sword. Polar Ray just isn't worth its spell level compared to Disintegrate, Chain Lightning, Cone of Cold, etc.