thats silly way to look at thing.
its an ability that exist in the champion kit, it should not be garbage ability to start with (aka it should not be a garbage version of enchantress ability) unless you want to over buff lux E/Q to make up for the fact her W is just a garbage version of enchantress abilitys
besides she is not the only mage with a shield comparable to enchantress , unless orianna count as enchantress she over more or less a comparable shield .
its not like lux outclass enchantress at what they do, she does not have enchantress utility and on top of that they have poke of the same level of lux and most of those pokes are not even skill shots to start with (soraka E if maxed deals 230 damage + 40% ap vs lux E which deal 250 + 60% and this is not counting soraka E being instant + silence + have a chance for double damage and root while lux only slow) .
the only reason to pick a lux over an enchantress is the range in laning and the scaling .