I get why otherwise, it would be an infinite mana generator
(Although I did have a bunch of Shadowless cards) As you said, there may have been a select few places where this was an exception, but I think peoples view of just how common 1st ed Base cards are is heavily distorted because of the prominence of 1st ed Base in the high-end collecting world
Heres the big part of the ballgame: overloaded, it gets around Gaddock Teeg
Because paying X is part of the normal casting cost, paying an alternative cost for these cards will result in X=0 unless the alternative cost itself specifies a value for X, such as Lantern Flare or Disrupting Shoal
I dont think well ever see any of the big cracking operations pop up with a couple hundred copies of any Oil-Slick mythic, and once the current wave of bundles is gone, thats probably most of the inventory that will ever be available