Here are two short paragraphs from the introductory chapter of A Comparative English-German Grammar by Elias Peissner (Schenectady: G. Y. Van de Bogert, 1853).
Vowels (and diphthongs) will sound a little more open before certain consonants. The cause of it lies in the nature of the transition from the vowel to the consonant. Compare, for instance, the sounds et, it, ut, üt, ot, öt with er, ir, or, ör, im, um, om, öm, in, un, on, ön, ong, ung, ink and you cannot but see the change.
Therefore learn only the primitive sounds well and leave the rest to nature. If you should learn thus to pronounce German more purely, than we ourselves pronounce it, think not that we blame you for it; no, no, we rather admire you and are ourselves ashamed of our carelessness.
No linguist would say a thing like that today, and especially not one who writes in English. English linguists have accepted the principle of description-not-prescription as a matter of religious dogma. Webster’s Third New International makes no distinction between the pronunciations of bidder and bitter, which sounds like giving in to utter carelessness. (Interesting to note that the Merriam-Webster web site has retreated from this extremism.)
Because of this dogma, we make learning to read a dreadful chore for our children, because we insist that they learn to divorce spelling from pronunciation, making the written symbols an utterly arbitrary code to be memorized, rather than transcriptions of a small number of sounds whose universality makes learning to interpret them a simple matter. This is the secret of the alphabet, one of the most marvelous inventions in human history.
We are also accelerating the already dizzying pace of change in our language. If we teach careless pronunciation as the norm, then of course carelessness will deviate from that norm, and that will create a new norm, which in turn will have to be taught as the norm, and so on. The English language is already changing at a more prodigious rate than at any time since the Tudor age; is it really necessary to accelerate the change?
Dr. Boli, however, has made an observation. What Prof. Peissner said about Germans is equally true of American English-speakers, as long as we except professional linguists and lexicographers. If you learn to pronounce long and short vowels carefully, and make clear distinctions between consonants, you may not sound quite like an ordinary American, but Americans will think you sound better than an ordinary American. If you are in charge of teaching English to children or other foreigners, you might consider that observation and do your students a big favor.