Are flat-field frames necessary?

That depends. Flat-field frames are the hardest calibration to do correctly; in some cases, you can get by without doing them, but at some loss of performance.

If you are doing photometry, you absolutely need flat-field frames. If you are doing "pretty pictures," then it depends on what you are doing. If you have bright skies due to light pollution, you'll probably want to flat-field. If you have vignetting, then you'll want to do it. If you have dust spots, a can of compressed air is actually more effective!

If you are going for the faintest possible limiting magnitude with a long exposure, then the limiting factor is caused by the fact that your target is fainter than the sky background (even at a dark site). In that case, the flat fielding error will set the ultimate sensitivity limit.