That way, Poloniex will not scare others when they tell people not to use an expiring address. I think that it is impossible for an address to expire anyway, don't you? But newbies do not know that.
At a protocol level... that is correct, addresses do not "expire".
However, some services/exchanges use "one time" addresses for receiving deposits. Often, once a deposit has arrived, they no longer "actively monitor" that address... so if you send a 2nd deposit, it won't automatically be credited and you need to contact support to get them to manually credit you.
Also, you have retailers using BitPay, which usually requires "seeing" a transaction broadcast within 10 minutes... and that the transaction is confirmed within a set time frame or the entire deal will be void. To really complicate things, BitPay will generally only send funds back to the address from which it was sent (if a transaction is voided), so if you send directly from an exchange to BitPay and it "times out", BitPay will want to send the funds back to the exchange itself (and not your deposit address) and then you're stuck trying to deal with the Exchange support team to recover your coins
The payment request facility within Electrum is part of it's
BIP70 support... which specifies a time at which the payment request should be considered "invalid":
...
expires Unix timestamp (UTC) after which the PaymentRequest should be considered invalid.
...