Recording a band at once is a cool way to create a lively "band" feel to the music. When musicians are all recording together, a certain "feel" or "groove" can be created. Some of my favorite recordings that I've done, were done this way. The inconsistencies kind of become features, adding to the vibe of the track, provided the arrangement and musicians are competent and relaxed. Recording live in the studio does demand a higher level of musicianship from a band, since serious mistakes made by one musician may affect all the others.

Doing multi-session recording on the other hand, allows the band to focus on one part at a time, which I guess is a little less pressurizing. In this way, more attention can be paid to getting each part right, which will ultimately result in a more "accurate" performance, but at the expense of sacrificing some of the organic "band" feel. Certain music styles actually benefit from a more "clinical" sound though, so I'd assess the sound we were going for, and make a decision based on that.