Which exercises and/or videos are you referring to exactly where the instruction said to use the mx components when spark components were available?
If you could let us know exactly which exericses or videos you are referring to that would be helpful for us to answer to your question.
Day 2, Episode 1:
Code contains mx lables and mx textAreas, but the spark versions of these controls are available. I think there are more examples, sorry that I don't have time to go back and check. I was wondering if the spark controls are not ready for prime time, eventhough I have had good luck with most of them so far.
Even with some of these concerns - the presenter and material of the FIW are top notch and I really appreciate the effort!
CTS Instructional Designer - UCLA