"3 out of 4 answers say it's bad. Why? or why not?"

For one because if you blow the fusible on the mobo to the USB power I will charge you fifty bucks to replace it, more if it is a laptop.

The thing here that batteries charge at a much higher rate than they used to. The old NiCds would charge slowly, at a low current rate, generally you had to charge longer than you could run. But newer battery technology has them charging at a much faster rate which requires more current, making that current drain significant.

Say, in the old days (yeah right) a batteryu might charge at 75 mA and the device needs 900 mA to run. The 75 is less than 10 %, not a big thing. But by the laws of physics (thermodynamics believe it or not) if the battery charges in say 2o minutes and will run an hour on that charge, the charging current must be 3 X the drain current. A battery, except for dry cells and that weirdo machine that suck magnetism out of PMs, is noting but a storage device like an electrolytic capacitor. In fact the operation is replete with similarities.

Let me try to draw this; (I suck at it)

Power - \ --- device
...........--- current regulator/logic --- battery

The "bottom line" there used to be insignificant, but now it can actually be MORE than the device pulls to run. A USB port might not be able to handle it.