With a regular CA, you pay upfront, and you can revoke and reissue with no problems. If you trust a CA that does this, you can count on the fact that anybody buying a cert from that CA has the ability to grab a brand new certificate whenever they want until the expiration period.
But with a CA like StartSSL, they give out certificates without guaranteeing that websites using those certificates have the ability to regenerate their certificate. If a website gets compromised, since they haven't paid upfront, you don't know if they will pay to reissue. They might just keep quiet and pretend there was no security breach, instead of doing the proper thing and renewing the certificate. If you know they paid upfront, you can be more sure that they will take advantage of the free renew upon a security breach.
I find this issue to be vaguely similar to the difference between socialist healthcare systems and privatized healthcare systems. If everyone is guaranteed to have healthcare because it must be paid through their taxes, then people are generally healthier since they are encouraged to go to the doctor. But if you must pay a bill whenever you receive healthcare, people might be encouraged to forego treatment or checkups that could catch deadly diseases in an early phase before they become life-threatening. I don't really think this is a good analogy though because the other pros and cons of both healthcare systems don't map well to this problem.