There is little else that is as unproductive as showing up for a rehearsal unprepared. Depending on the size of your group there are anywhere from three to five or more other members counting on you to show up prepared to run through the songs. It is one thing to have a momentary lapse in memory or concentration and entirely another to show up to rehearsal with the intention of learning your parts on everyone else's time. This is completely apart from writing or composing.
For me, practice is what I do at home in my studio to prepare for a rehearsal, show, or audition. In fact I view each rehearsal as a personal audition - it keeps me on my toes and up on my material. So what does practice entail for me? Almost exclusively I practice using my laptop, Focusrite interface, and headphones. I warm up with a few scales and fiddling around for around five to ten minutes depending on how much time I have on that particular day, then I get right down to it.
I start off by going through any new material we are working on and any problem areas I may have on any of the songs. There is a set list for each of the bands I play in. Typically we vary the set-list from show to show. I play through each set list in the order that it will be played for the upcoming show. By keeping up on the material I find it is easier to incorporate new material to the set-list. I make a special effort to exercise critical listening and critique my playing. I find that using headphones really helps me to isolate certain aspects of my playing in order to fine tune what I am working on. This enables me to be prepared for rehearsal and not waste anyone's time.
Bottom line is that it is hard enough to get everyone in the same room together so time is a precious commodity. Anyone who chooses to use rehearsal as practice is stealing both time and progress.
