[throwaway76543]

It's fine if it's used to simply pull down mail to a local spool. The inefficiency complaints usually stem from attempting to maintain mail on a remote server across multiple clients. POP3 does not implement a sufficiently rich protocol to make this possible without downloading the entire spool onto each client.

For example, no searching.

[glandium]

No, it's highly inefficient in the face of latency, because there essentially is no batch fetching primitive. Fetching messages over a high latency connection (e.g. to a server on the other side of Earth) takes forever if you have a lot of messages.

[throwaway76543]

That's addressed by pipelining (rfc2449, published back in 1998). No batch primitive is necessary if the request/response sequence isn't serialized.

The lack of a remote manipulation interface has always been a problem, however.