A consent form is important because you're usually recording the person's voice and sometimes their face. They may have concerns about a video of them fumbling through a product being published online, so gaining permission to record them, and reassuring them in writing that the audio/video will only be shared within the project team is important. Also, many people don't realise before the session begins that they're being recorded, so having them sign for the fact clearly articulates that they're agreeing to it. It's akin to a model release form in television or film.
I agree that it shouldn't be something that takes any time (those templates I linked to above are good enough). But it shouldn't be skipped!