In traditional software development, if you were programming in a framework such as .NET 2.0 and a new framework was released, such as .NET 3.5, you'd go through a painful upgrade, where you'd have to reapply and thoroughly test your code.
Now, with the new PaaS/ SaaS offers, such as Salesforce.com (SaaS) and Force.com (PaaS), each major version upgrade is usually made seamless by the vendor (Salesforce).
Do you think not offering seamless upgrades, i.e. upgrades where the users have to test their applications and eventually correct them, is a big deal breaker that kills the UX of the customer when using the PaaS/SaaS service?
What is the advantage of offering seamless upgrades?