About retrieving iLvl by scantip

[siweia]

For those items from timewraped instance or heirlooms, most of the gametooltip addon try to retrieve iLvl by creating a scantip. Same thing happens in libStub,

scanningTooltip = _G.CreateFrame("GameTooltip", "LibItemUpgradeInfoTooltip", nil, "GameTooltipTemplate")
scanningTooltip:SetOwner(_G.WorldFrame, "ANCHOR_NONE")

Why don't they retrieve all the item levels from this scantip thing?
They won't have to update the upgrade table anymore if do so.
What's the disadvantage of the scantip?