Spellcasting is a class ability.
Honestly, I always found it a bit strange that a race known for deception and arcane magic would have a low Charisma. Thus, they are not very good at Use Magic Device, despite being magical, infernal beings and having a reputation as rogues. They are also not good at commanding undead or bargaing with fiends. Much oddness. Wouldn't instead giving them a racial penalty to Diplomacy have skipped to the point?
D&D never makes sense with the Charisma penalties. Mindflayers are apparently hot since they have high Cha, but Dwarves and 1/2 Orcs are not so they get penalty.
Spellcasting is a class ability.
Honestly, I always found it a bit strange that a race known for deception and arcane magic would have a low Charisma. Thus, they are not very good at Use Magic Device, despite being magical, infernal beings and having a reputation as rogues. They are also not good at commanding undead or bargaing with fiends. Much oddness. Wouldn't instead giving them a racial penalty to Diplomacy have skipped to the point?
I personally think that the Fiendish Sorcery is a really cool racial ability. It gives more to their heritage than just a couple stat bonuses. One of my players loves Genasi so I converted them to Pathfinder for him and definitely included this change (obviously for elemental bloodline though). I'm also pretty sure its gonna show up with Aasimar in the Bestiary.
Overall I do agree with removing the negative cha, although I think with the Fiendish sorcery the adjustments should be: +2 dex, +2 int, -2 wis. This gives them Rogue, Bard, Wizard, and Sorcerer as viable classes.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.