Well, for someone like me, if Verizon (my provider) discovers that I've jailbroken my phone, they can and probably will invalidate my contract with them. Since I have a service offered through them that they no longer offer - unlimited data - I'm not going to risk losing that by jailbreaking. Could I go to another provider for that service, like Sprint? Yes, but it would be a huge hassle, and besides, my out-of-town parents also have Verizon service, which means I can call them anytime without being charged minutes to my phone.
I just use Skype for video calls. It's a bit choppy on 3G, but it's sufficient when I'm not in hotspot territory.