First off, wouldn't it generally be a 3rd level spell? Equivalent to Sorcerer 6 is when you get the spell, and you can't change the spell later. Though I guess nothing prevents you from choosing an abnormally high level spell that you won't get access to for another 8 levels or whatever.
Anyhow taking legacy champion+Uncanny Trickster+Bloodlines is not always an option, not to mention fitting those into a decent build. Plus, you require another PC or the leadership feat to make good use of the ability without relying on the DM simply giving you a dragon that knows the spell you want. So it's a decent trick, but not that broken given the heavy investment.
How about: Kobold Abjurer 3/M. Specialist 10/Initiate of the 7-fold Veils 7, with a Dragonkith cohort that is a desert orc paladin 6/Intermediate Gold Dragon Bloodline 2/Dragonkith 9? You could ride on your buddy's shoulder and share all your tasty abjurations with him. Plus, desert orc fills the Endurance requirement, while dragon bloodline grants alertness, and your orc can be riding a dragon, this creating a megazord of draconic creatures. Plus, it gets that 1 5th level SLA, castable when the cohort hits 17th level.