Several other skills are also useful even in those upper reaches. Disguise, for instance. A mundane disguise is undetectable by magical means, and opposed by a Spot check (which can vary by level). It's on the Bard's list, but not on the Sorcerer's. Hide is also opposed by a Spot check - Bard has it, Sorcerer doesn't.

Though for extra cheese, there's no reason you can't just take one (or two)levels of Sublime Chord, then another PrC that advances a spellcasting class. Just specify that the class you're advancing is Sublime Chord.