Freedom to install any random software I want whether or not Apple approves. Freedom to use AT&T's (or whoever's) network only worrying about how much data I'm transmitting or receiving, not whether or not AT&T likes what the data is.
First, that's possibly illegal. I imagine Apple tolerates it without invoking the DMCA because they realize it increases the popularity of their phone and people can use the ability as an excuse. I don't consider doing something possibly illegal to be an option.
Secondly, I want everybody to have that freedom. Many applications increase their usefulness by having more people use them. If people have to go to inordinate lengths (like doing something that already sounds pretty questionable) to use the same application I am then that means the application's usefulness is limited for me.
But, I already know I don't want an iPhone because of the DRM. Whether or not jailbreak violates the DMCA isn't big on my radar. But, a bit of preliminary research shows that nobody really knows for sure. The way the DMCA is written is very vague and it's difficult to decide what does and doesn't violate it, especially in the case of things like jailbreak.