I beg to differ that the SEP client is of use. How will you protect the OS from viruses that exploit security holes in the OS if the OS is not patched properly (and for that matter, when an exploit is determined to the amount of time it takes Microsoft to develop an update)?
If SEP goes down (assuming you mean SEPM), you can edit your management server list to fail over to another server. If your server crashes due to Spyware/Viruses, then I would think having the SEP client installed is one more layer of protection to prevent that scenario.
SEP is not designed to scan Exchange POP3/SMTP emails, that's what other products are for. SEP is there to protect the OS, it's absolutely of use!