Joker555 has I think answered your message perfectly. Silk may not be viable at all to evolve for her iddle gain (and it's far from the only useless girl on this list). Her strength comes from her extremely powerful Perks (x15 iddle for the 3 categories of girls).
Having girls with different uses seems a good thing to me. Especially since it's a Gacha Girls, see her like a big passive bonus (well if you put the necessary affection of course).