Making them very hard to come by in any condition let alone mint raw with a holofoil pattern that may very well be more susceptible to damage than any other foiling method
Also when you are concerned about early tempo you are also likely going to be concerned about life and so not having the painless cast option does detract from the appeal of the card
A lot of green decks are going to find ways to modify creatures (using the definition of the mechanic modify, of course), but in a pinch you can use Silkguard to modify them anyway
It does so by: Killing things efficiently Summoning powerful/under-costed creatures Drawing extra cards Using the graveyard as a resource Forcing the opponent to discard key cards Top Cards These are some of the best black cards in today's competitive Standard
Similarly, if you cut cards like Top and Library of Leng, Urza's Saga effectively does nothing
Realm-Scorcher Hellkite Realm-Scorcher Hellkite is a strong 6-drop dragon, allowing you to have a 4/6 with haste that, if bargained, produces 4 mana