Buying training software/videos for the application software you are purchasing: is it a good idea?
Well - if your instructor is well versed in the entire package then the answer is "no".
However, so often the case is more like this: your instructor is really knowledgeable about Dreamweaver (i mean a guru! could have written a book) and relatively competent with Flash (has used it and likes it) but not so much with Photoshop or InDesign. And you just bought the CS3 Design Premium licenses for the lab she is teaching. If your instructor does not get training on these products then you wasted your money. Because without the training the software doesn't get used (or taught) and you might as well have spent your money on something fun like Wacom Tablets. Or pie.
That said - there are a variety of products created just for training. AND there are many products that offer FREE training - you just have to look for them.
Tons of products: Total Training offers a complete line of titles for learning pretty much anything and everything Adobe, lots of things Microsoft, with a sprinkling of other publishers as well. They offer training CDs/DVDs on single titles, on suites (such as the CS3 Suites), 1-yr on-line subscriptions (minimum 5), and K-12 Site licenses on a few of the CS3 Suites.
Adobe products: Firefly Learning offers training videos on Photoshop and Indesign.
Quark offers free training downloads from their website.
Corel offers free training built into there software. For example: CorelDRAW Graphics Suite 4 has 2 hours of training.
Bottom line: Training software is cost effective. Training your instructors is the only way to really get your money out of your software investment.