All or none is dumb. The only reason you have to ignore any is that the spells don't work well with some of them, to the point that enforcing the games point limit on summoned units is ridiculous. That's only justification to igmore those some. 'This shooting rule is broken, therefore we must ignore the entire magic phase chapter' is dumb reasoning. 'This magic item doesn't make sense in the rules, therefore its entire faction cannot be played' is dumb also. 'All or nothing' are no less arbitrary places to draw your lines, and in this instance both have absurd results and thus are bad places to draw your line.
GW does not write tight rules. They just don't. So all interpretations must pass through a 'does this make sense / is it reasonable' filter. summoning spells being inherently useless because you already purchased you're full game allowance during List construction isn't reasonable. summoning spells duplicating unique heroes or 1 per army options also is not reasonable. Much as rendering default harbinger useless by letting the sig spell summon characters also isn't reasonable.
I would love for gw to produce tight rules that can be read as raw at face value, but they don't. Go down that road, and you have undead legion tomb kings that cannot join chariot units, undead legion characters in general with no access to parent book items, and so on.
But whatever. If you want to be that way then fine. Rules are in effect unless something else, some other rule, says they're not. It is on you then to find rules text that states summoning ignores uniqueness. The summoning rules say the options that you give summoned units must be legal, and it is not legal to give a character a magic item that your army already has.