I agree with some of the recent posters, this argument is getting silly!
Last comment I'm gonna make on it, and here's why I don't buy the "She has to do a nude scene to challenge and stretch herself as an actress" line.
If "challenging youself as an actress" involves doing something a little scary, bold, and something you probably don't want to do, then what's so special about nudity?
Why not say they have to eat a plate of scorpions, or parachute out of an airplane, or any other number of "Dog Eat Dog" type of stunts in order to "challenge themselves as an actress". Why is nudity such a benchmark and measure of challenge?
No it doesn't.
I'm ignoring your other comments because I've already explained why I think it does show character when considering her commitment to a role. Oh and yes the Machete script was changed to fit her requirements.
Commitment to the role? What's so hard about taking your clothes off? All it shows is that she's willing to get naked. By that argument Porn Stars are the most committed of all actresses. I think there are better indications, such as research into the part, physical changes (I'm thinking of Dinero in Raging Bull, but also maybe things like getting your tongue pierced, etc).