Fair point.
Most of the classes I drop in on are large, lecture based classes, where I'm going more for the lecturer than for the validation.
In some cases (CS courses), it's really the peers/TAs that matter more than the lecturers. Of course, gaining trust and access with them is trickier than just showing up to a lecture, but it's definitely not impossible :)
I'd say the hardest part about attending a lab session would be getting a computer account (assuming the computers are networked). I'm sure I've seen students that don't attend lectures (presumably looking at them at home) but are there in the lab sessions.
In some cases (CS courses), it's really the peers/TAs that matter more than the lecturers. Of course, gaining trust and access with them is trickier than just showing up to a lecture, but it's definitely not impossible :)