yes mabe "never never never" is a bit extreme... If your +5V supply is regulated well enough it may work. In my expereince though, you avoid a lot of hassles by not putting inductive loads like steppers and solenoids on the logic supply! What I should have said is that if you use separate supplies -i.e. one for logic and another supply for messy inductive loads- you can pretty much be sure that the indutive will not interfere with the logic supply. It's one less thing to worry about.
Re the second question, you're probably geting 8V or so at the Vin connection because there is a blocking diode (D1) that drops .7V of the supply voltage. Most "wall wart" supplies aren't goign to be exactly 9V either, as they aren't usually regulated.
D