There are two things that people jailbreak their phones for: Running software that Apple doesn't want on the phones (backgrounder, some emulators, tethering) and running software that was downloaded/purchased illegally. Honestly, such a small minority of people jailbreak only to unlock their sim cards that I won't even consider them.
As to the former: Fine, have your slowed down devices, play your roms (also illegal, btw), violate your service agreements, that's on you.
As to the latter: You are stealing. You are stealing food from the tables of the developers, and they don't like it. If you keep cannibalizing the talent on the app store, it will cease to create stuff that's interesting.
I wrote a post-mortem on my blog discussing some of this stuff:
http://www.kraln.com/?p=108. It really makes me sick.