Yeah, I know the type. You have to lay it out flat. Just tell them, if you're into this as much as you say, then we gotta practice. As far as jobs go, that's a pretty good excuse. Until you ARE famous, you can't just skip out on work to have band practice. Unfortunately, it doesn't work that way, especially if you have a real job.
And what your singer says, I kind of agree with. When I played guitar in a band, practicing at home didn't help much. Practicing with the band, however, was so amazingly helpful. And I did the metronome/drum machine stuff. It's just not the same.
So basically, you just gotta let 'em know what's up. Especially about the dedication thing. Our old drummer, the first time we practiced (we got his name out of the local music paper, so we tried him out), he said "if we're gonna do this, I have to take it seriously. We have to practice more than once a week." And the three of us just thought...cool, this is perfect. Then we parted ways for some reason, I think it was because he kept cancelling practice. Then he got an offer to play a show (he was friends with the booking guy at a local bar)...so we did. And instantly, we all agreed to play again. Then we played another show (same place), still not officially a band again. So we got back together, played twice a week, rented out monthly rehearsal space...then he broke his bass drum head. He said "practice is cancelled until I get a new head...I'll call you when I do." He never fixed it. What dedication, right?
So we kicked his ass on the street...and then I picked up drums.