In my opinion... The client needs to make sure rogue servers don't pick up their bits and the server needs to make sure rogue clients aren't spoofing real ones. From this standpoint, IPsec is a bit overkill because, as Chris L. pointed out, there are situations where the server wants to receive information from unauthenticated clients. Also, the ideal logging system would have the message authentication/encryption capabilities follow them onto stable storage, whereas IPsec is a purely on-the-wire system.
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