This has come up before, and my opinion on it (http://paizo.com/threads/rzs2tppf&page=2?PFS-Character-Retraining-rule s-a-bit-over-the#60).
As has been pointed out elsewhere in this thread, the way the retraining rules are written opens them up to being able to create characters that are otherwise impossible. (eg. a second-level magus with power attack)
I doubt that this was the intention - it goes against one of the issues with D&D3.5 highlighted by Jason Buhlman in 2008 - that characters (especially multiclassed ones) were impossible to audit.
Given the potential for exploitation here, the reaction has been to limit this by making the costs for retraining "approximately suitable" when attempting to abuse the retraining rules in this way.
As a result, retraining poor character levelling decisions is prohibitively expensive.