Is it alright to use “usen’t” instead of “didn’t use to”?

Samuel Beckett also uses "usen't" in Embers:

Henry: I usen't to need anyone, just to myself, stories, there was a great one about an old fellow called Bolton, I never finished it, I never finished any of them, I never finished anything, everything always went on for ever. (Pause.)

Good enough for him, good enough for anyone.

I agree that "usen't" is inappropriate in formal writing. However, it is found frequently in literature, though almost exclusively in dialogue (or stream of consciousness monologues). For example:

In classical literature:

• John Galsworthy, in The Foresyte Sagar: You have changed, you usen't to have that line between your eyes, and your jaw's getting too strong.
• Oscar Wilde, in The Importance of Being Earnest: LORD DARLINGTON: I am not one of her asmirers. CECIL GRAHAM: I usen't to be, but I am now.
• George Bernard Shaw in Major Barbara: That is a new accomplishment of Andrew's, by the way. He usen't to drink.
• A.J. Cronin, in The Citadel: Don't make a joke of it, darling. You usen't to talk that way.

20th century novelists:

• The Hireling, by L. P. Hartley: He doesn't take taxis much - at least he usen't to.
• Never Again, by Francis King: It usen't to be like that. We used to see much more of the boys- my husband and I... It was more a family, then.

21st century novelists:

• The Stories, by Jane Gardam: 'I'm slow too.' 'You usen't to be. I'm sure you could write a big book.
• Sheila O'Flanagan uses it in dialogue (for example, in My Favourite Goodbye: I usen't to think so. But I do now.) and in stream of consciousness monologue (for example, in If You Were Me: He usen't to be a morning person, but maybe he's changed.)