Yes, you could use it without batteries and with a wall plug adapter (I think), but it would really suck if you needed the firearm and the power was out... Also if the power goes out without battery backup, the code will revert to the stock 1234 code.
So yes, the power adapter would work well, and with battery backup you should get a REALLY long life out of the batteries because it won't be constantly drawing from them (assuming the wall power overides the circuit for the batteries).
Good luck. I'm thinking about getting one myself.
Regards,
Cedric