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. 

zapsammy

Stacker Saloon

saloon

interesting

>"to obtain exemption or release from an obligation or duty; beg to be excused" is from mid-14c.

>The weakened sense of "excuse" is attested since 1912, but technically any proof of innocence that doesn't involve being "elsewhere" is an excuse (n.) and not an alibi.