What if the person is on the subway for 20 minutes? instead of getting the SMS when they get out theyll get a SMS because its been longer than 5 minutes and I will be charged for an SMS? I hope not.
What if the person bought a new phone and iMessage isn't coming back online for that number? SMS is a standardized fallback that will work with any phone.
This may seem extreme, but at least during the beta, many users are downgrading back to 4.3.3 because of battery life. Without an intelligent fall back, the message could be lost.
BTW - If your SMS plan is an issue for you, you could send your iMessages to their email address instead of the phone number. If you do this, it can not escalate to SMS.