Same reason the PayPal app on iOS says not to use it if your iPhone is jailbroken. Removing the security layer allows for effectively uncontrolled inter-app interactions. Sandboxing and runtime restrictions are pretty much gone.
Originally Posted by WorldIRC
Unlocked iPhone 5, unlocked iPhone 4
Unlimited airtime, Unlimited CAN/US long distance, Unlimited SMS to CAN/US wireless numbers
2500 Call Forwarding minutes to CAN/US numbers
Google Voice for visual voice mail with message transcription, conditional greetings, unlimited messages (vs 35 message cap), remote retrieval from any PC or phone, no auto-purge after 10 days and most importantly no $7-$8 charge.