In terms of assertion, you can be very assertive using "watashi" while still being formal. (Euphie is incredibly assertive using "watakushi".) "Ore" is considered borderline rude in some situations, simply because of how informal it is. Selfless Suzaku uses "boku", while the more self-absorbed Suzaku uses "ore". For Japanese social norms, Boku!Suzaku looks a lot better and represents better values than Ore!Suzaku.
In English, we really don't have the sort of formal/informal self distinctions, but something comparable might be using contractions. In a formal speech (like a presidential address or something), you don't generally see a speaker use many contractions. However, in casual speech, they're everywhere. It's not necessarily wrong to use contractions in formal speaking, but it's generally not done. Or if it is, it's very sparing.
Considering Japan/Japanese's insanely strict social etiquette, it seems more likely that being so informal as to use "ore" in Zero's position would be out of the question. And, as Lelouch has said more than once, Zero is a symbol. Better to play to the loftier side of the image with more formal language. He also uses "shokun" instead of "minna" to address groups. It's like using "ladies and gentlemen" as opposed to "everybody" or "you guys".
(no subject)
In English, we really don't have the sort of formal/informal self distinctions, but something comparable might be using contractions. In a formal speech (like a presidential address or something), you don't generally see a speaker use many contractions. However, in casual speech, they're everywhere. It's not necessarily wrong to use contractions in formal speaking, but it's generally not done. Or if it is, it's very sparing.
Considering Japan/Japanese's insanely strict social etiquette, it seems more likely that being so informal as to use "ore" in Zero's position would be out of the question. And, as Lelouch has said more than once, Zero is a symbol. Better to play to the loftier side of the image with more formal language. He also uses "shokun" instead of "minna" to address groups. It's like using "ladies and gentlemen" as opposed to "everybody" or "you guys".