I used to wonder about this too.
But apart from using the F5 across the WAN (For which there's a module), where else would you want to incur the overhead of compressing and decompressing the traffic between the LTM and the servers? The whole idea of LTM's is to reduce the load on the servers. Not increase it by adding complexity to the data they have to handle
OK. Maybe there's are 1 or 2 borderline situations where the servers have oodles of CPU but saturated network links... In which case I say why don't you add server network interfaces... (e.g. etherchannel)
H