Combined Spell Immunity and Resistant Skin and Invulnerable

I see that some units in WC3 have got both Spell Immunity and Resistant Skin, and some have got both Spell Immunity and Invulnerability. Is Resistant Skin necessary when it has Spell Immunity? And is Spell Immunty necessary when it has Invulnerability?

Resistant skin protects against ultimates as well (makes them treated as a hero for duration, and targeting). Eg: cannot be charmed.

What about Invulnerability? Does something even damage that? (I mean in melee games, not in triggered maps)

some friendly spells can effect invulnerable units...for example auras and buffs. in other words magic immunity is a disadvantage here.

Thanks Captain and SFilip, would give you rep but the forum system doesn't allow me ;-)

Not only ultimates, Resistant Skin also protects against non-magic skills. So, on a unit with Resistant Skin, Ensnare will only last as long as it lasts on heroes rather than as long as it would last on a normal unit.