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 [I]is[/I] 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!