The perk to buying the extra skills is the price for the Power that Magi and Psions possess. The perk is that they can cast Spells and Talents instantly that have a 1 day duration at line of sight. No more than that even needs said, but you know me. The only reason they got the *perk* wasn't because they paid more XP for the skills to cast, it was to balance out the fact that fighting styles gave bonuses to Attack, Will, Inits, and Dodge. Meaning a Magi going off of Discipline + Casting was capped at a 2d6 + 36 vs a Will, Dodge, or Inits of 45. Now that Fighting Styles, Dodge, Inits, and Will follow the same caps, Magi no longer need this bonus to keep themselves balanced, so now it's gone.
I think Instant cast masteries with day long durations and line of site range is a perfectly decent compensation for having to buy two additional skills at medium cost.