1. QM was created because of the problems of particle physics. Sub-atomic particles don’t follow the rules of macroscopic particles. It was what was called the “Ultraviolet Catastrophe” that is generally regarded as the basic problem QM was needed to “fix”. Also known as the Blackbody Radiation Problem it regards how a perfectly emitting body would emit radiation. Classical Physics said the black body at thermal equilibrium would emit energy with infinite power. QM was developed and said it was more like E = T exp 4.
QM was right and rocked our world! There are too many other examples to list.
2. The uncertainty principle is an attribute of nature. So as to defending its existence – it is a real phenomenon -- we have little choice to accept it even if it doesn’t do anything some would consider “practical”. The Casimir–Polder force is an example of the uncertainty principle at work.
3. Theories attempt to make sense of observations to the point that we can make practical predictions about future events.
4. Justifying something is a rather vague measure as, among other things, it is totally subjective. In science the established fields have agreed on methods and measures to move together in a meaningful way. What the rest of the world does isn’t “science’s” business and it likes it that way.
5.Two Scientists in 1971 Cluaser and Freedman did the seminal experiment to answer that question. They determined (many times confirmed) that to the limit of statistical predictability the predictions of QM are correct to within 3% (their limit).
A.While QM seems to be an easy target because of the many non intuitive (read:way -- strange) features. This is not QM it is nature itself. In reality QM in the form of QED, for example, is almost unimaginably more precise than any other human construct. The QM theoretical magnetic moment of the neutron compared to the actual measured magnetic moment is correct out to TWENTY decimal places! It is the most exact theory that has ever been created.