yeah I can think of a reason you would want to send an SMS over an iMessage:
you're on your iPhone iMessaging someone on their iPad (or iPod Touch) all well and good, then they have to go out, obviously they leave their iPad at home and pick up their phone to take with them. (it doesn't even have to be another mobile, it could be an iPhone but they don't have data turned on).
If you then want to message them you know you would have to use SMS but it will want to send it as an iMessage as their iPad is connected to their wifi at home.
as I understand it this would be a problem if the iMessaging email address and mobile number were under the same contact. so splitting the contact in two would solve the problem (but splitting a contact is hardly ideal).
has anyone tried, say, long pressing on the send button when it is in iMessaging to see if it gives you different sending options? or trying out this scenario out?
you kinda don't make sense with that scenario, if you're messaging them on their iPad, then its going to an email address. if they then take a phone out, you message their mobile number..... it's 2 seperate things in that sense. for example, the other day, my mate has iOS5 on his ipad, that was one convo, then he text me from his mobile, that was a seperate convo in the messages app.