Without watching the entire video...
2 megapixels MAY be ok to print a billboard, but that's because most billboards are meant to be viewed from far, far away. When you look at digital billboards, most of them are very low resolution. However, there are a lot of other types of large sized printing -- like wall and window decoration, bus wraps, or posters -- where people can come right up to it, and it's important that it looks good from 2 feet away.
But anyways, I have to agree with the main reason that most everyone has cited here, the need to crop. It's especially true if you like taking photos of little wee birds like hummingbirds or chickadees or bushtits, because otherwise, you'd need a monstrous lens to shoot one at twenty feet and fill the frame.
And even so, birds in flight (and I suspect a lot of other potentially unpredictable movement photography) usually need to be cropped; they're normally shot in a larger frame and cropped down because that's far preferable to accidentally lopping off part of the bird
Most of us who aren't pro photographers will generally tend to shoot a little wider and crop anyways (better safe than sorry). Heck, I even cropped in the days of film and enlargers. And for studio photography, there's often a lot of crop -- to get rid of the lights and all that, since lighting can often be very close to the subject. Plus, the lens you're using and the position you take is often based on the perspective you want and the distance you have available -- which might leave you with crop requirements.