Familiaar-beschaafd.
Nothing is more mistaken than the view which is sometimes taught, that the colloquial style is less ‘correct’ than that of books, and that such contractions, for instance, as isn't, can't they're (they are), I've, he'll, and hundreds of others which are habitual to all good speakers of English, are in reality vulgarisms, which ‘correct’ speakers should avoid. The fact is, that these forms are in many cases the only ‘correct’ forms in colloquial speech, and to use is not, they are, he will, and so on, would be pedantic or worse, if that be possible. Whether it is at all times suitable and convenient to use these colloquial forms in public speaking is an entirely different question, and one which the good taste of the speaker must decide, with proper regard to the occasion and the audience.
The use of literary or semi-literary words and expressions in colloquial speech is often a worse offence than the use of colloquial expressions in public speaking or in writing. Good writers know by instinct just how far familiar expressions may be introduced into a literary production; good speakers feel, in the same way, just where to draw the line, in colloquial speech, between what is expressive and appropriate on the one hand, and on the other, what would be pompous and affected.
H.C. Wyld: Speaking and writing in The Growth of English, blz. 95-96.