I just went through and made use of the 'skill classes' for my Sim very successfully in the following manner: I wanted to complete the Mixology aspiration, but my Sim had already gone the route of the chef in that career path, so there was no going back below the split-off in the career path to choose mixology instead. However, I was advised the only way, aside from cheats or mods, was to use the DU Sit In On Skill Class for Mixology 10 times. (I did cheat her skill level in Mixology first from 10 back to 6.) After sitting in on the Mixology skills class 10 times I received a notice that she got the 'Mixology Certificate' (and it had taken her skill up some, but not significantly, as I took the classes). After having her quit her chef job, and having received the 'Mixology Certificate', she was then able to check the available jobs and found a job available for a Mixologist'. She joined that job (at the 'Head Mixologist' level) , then was able to have the Mixologist career choice count toward that Mixologist aspiration.
This is the only time that I've tried out the 'skills classes' option at DU, but it worked out well for me, and it fixed my issue with career path opening, including getting a higher level career path job (level 6).
