Who says? The reason I use italic+underline is because (a) the reader can easily not notice the italicising when skimming, and may not have the best of eyesight anyway, and (b) when quoted, all text becomes italicised, and therefore stress would be lost without the underlining. Are you confusing fashion with some kind of ratified standardisation?
Yes, I use combinations of the font modifiers (they're available, why not use them - and I don't exclude underlining from the set of font modifiers) to achieve levels of stress or emphasis on specific words or phrases which would be stressed when spoken to achieve the desired meaning. Punctuation does not indicate stress, only pauses, and it is the pauses that I was talking about above (and conversely, stress does not indicate pause).
Okay, the next thing you will ask is why not just underline and not use italicising at all; I happen to think it looks softer.