both an alibi and an excuse are pleas. perhaps an alibi is given a stronger consideration, because it's about one party making a plea and another party providing the evidence, whereas an excuse is only one party making a plea.

recently i was suspicious of the word "please" - but it appears that it comes from "pleasure" as in "my will, my wish, my pleasure." so technically saying "please, ____" is an expression of a wish rather than a plea/request.