If you're going to look at that direction, you should also note that most security fixes come from the jailbreak community. After all, as they're finding vulnerabilities to exploit, this shows Apple where they've made mistakes. There have been HUGE vulnerabilities that could impact all iOS users maliciously that have been fixed for JB first and then iOS when Apple reacted.
You're also using a percentage to overstate the problem. 100% of unicorns live in Sweden. Does that mean we should all be taking a quick trip to Sweden to see these unicorns? Or, should we realize that 100% of 0 is still a pretty small number?
It's not wrong to point out jailbreaking opens you up to that possibility. It is, however, a stretch to word it as you did making it sound like a virtual certainty. The chances are minimally greater. For 99% of users, it's a non-factor.