I'm jumping on Dav's wagon here a bit....
Yeah, totally wrong.
Part and parcel of your practice time is learning those songs, and improving your playing of them. If you don't do it in practice, when do you? At band rehearsal? There are folks on this forum who would literally crucify you on your own basses if you tried that in a band with them, and they aren't simply a vocal minority. Rehearsal is for the band to work on the song. Practice is where you learn and perfect your part of that song.
The rest of Dav's advice is spot on, I'm simply going to add one more thing.
Get yourself a Zoom B1on or something similar, something that functions as a headphone amp, but will let you also plug in a recorder or somesuch. Record rehearsals. Remember that a key part of being a musician is your ear, and it needs practice too. Listen to yourself from the last rehearsal, did you botch that series of 1/8 note pulls offs in the bridge? Well guess what we'll be working on in practice this week?
And it's not just band courtesy, it's key to your development. Dollars to donuts says you'll recognize improvements in how you play those songs before you recognize improvements to the spider. And if you don't, your band mates are liable to, hopefully one pipes up... Don't underestimate the power of positive feedback, particularly early on. So focus on the songs more than the exercises up front, you'll know when it is time to start putting the exercises to the fore.
EDIT: Apparently I don't mean it when I type "one more thing"...