BIG-IP : SNAT necessary if device is only gateway to internet ?

I may be reading your question differently from the other responders. How is routing configured for your back-end servers? Does their default routing path send all traffic through the BigIP, such as via a default route or perhaps a self-IP is their default gateway? You stated, "have no route to internet other than through BIG-IP" which leads me to believe there is a means in place of routing traffic from the servers through the BigIP.

In this scenario, a SNAT shouldn't be required for the servers to simply respond to client requests to a virtual service. The BigIP will perform address translation from the VIP to the back-end server, and conversely for response traffic.

Now if the back-end servers initiate traffic outbound to the Internet (say, for updates or someone just wants to browse DevCentral), they will need some mechanism of address translation to have a publicly routed address to the outside world. SNAT is one method to accomplish this.