It is a risk. It is entirely possible that, if it is based on an older design, the CM has a lot of its amperage on the minor rails, and not enough on +12V to safely use an adapter. It is not something I would do with my own systems; others may be willing to do so.
Completely agree. People used to under estimate the importance of a good PSU, until they got some fried components, or stability problems.
I've always found what people vastly overestimate is the actual power usage of their machines to recommend crazy high PSU requirements, just like the marketing department of manufacturers want. http://www.guru3d.com/articles_pages/amd_radeon_hd_7850_and_7870_review,7.html
Total machine usage from the wall with a 7870: 274W (Assuming 90% efficient PSU, that's delivering under 250 watts to the PC. http://www.anandtech.com/show/5691/the-retail-radeon-hd-7870-review-his-7870-iceq-turbo-powercolor-pcs-hd7870/4
Total machine usage from the wall with a stock 7870: 310W (~280 delivered @ 90% )http://www.legitreviews.com/article/1967/11/
Total machine usage from the wall with a stock 7870: 284W (~256 delivered @ 90% )
All of them using rigs more powerful than his listed other components, and all well under 75% of his PSU's 408w rating on the 12v lines. Plus these are probably generous figures as their PSU's in the test boxes may not be 90% efficient.
That last one I linked was the only one using a 2500k (OC'd to 3.8Ghz), and was also using a 1000w 80plus bronze PSU, putting it somewhere close to 80% efficient at 25% of it's rated delivery capacity, so it's probably actually delivering more like 228 watts to the machine . . .