they'll always let you on if you do it at the gate. I've never had an issue with it (even changing to later flights, and even getting EUAs on them)

I think it makes sense trying to stick people who don't know better for the $25, knowledge is money

But I also agree that, if there is space, getting people who are at the airport on earlier flights is better. Once the person is off, they are gone, the airline doesn't have to worry, those seats would have been 'spoiled', while later seats could theoretically be sold. Further, in the event of unforseen problems, that is one less pax that must be reaccommodated.