Just because they're shot open matte doesn't mean the matted areas are meant to be seen. The pan & scan versions are NOT just opened up. T2, for example, has extensive re-framing ranging from zooming in a lot to opening up most. Also, all effects shots (CGI or optical) are hard-matted to at least 2:1. So, you're losing picture. Original prints of Aliens were hard-matted to 1.75:1, too. James Cameron always shot the films for the theatrical aspect ratio. He would at least supervise the pan & scan versions, but it's not like your ordinary open matte.

The Harry Potter films, as well as the Lord of the Rings films were all shot in Super-35, but since they had digital intermediates, the most they can be opened up to is around 1.78:1. Same thing for The Matrix trilogy. In fact, in The Matrix, there's a ton of mistakes visible when the mattes are off. You can see greenscreen that was never filled in, for example. Of course, any special effects shot is cropped a ton on the sides. A lot of directors won't even allow pan & scan versions on DVD. Take note on how the Pirates of the Caribbean films are widescreen-only, despite being Super-35. David Fincher shoots in Super-35, but Seven, The Game, Fight Club, Panic Room, and Zodiac are all without pan & scan except on TV.

But if you want it just for the nudity, you might as well go all the way and support pan & scan so your TV can be filled up.

As for other mistakes caused by not correctly framing for widescreen, Pee-Wee's Big Adventure has two shots ruined. You can see road signs being pushed along a track and a chain being pulled out of a hole. The Godfather has to be zoomed in a lot since there's boom mikes in nearly every shot if you take off the mattes. The Cable Guy is Super-35, but the entire film is panned and scanned from the 2.35:1 image.