I understand what you're saying about the similarities between CB and WR but I have to disagree. At WR you have for-sure starters, at CB there are questions after Grimes. There's no telling if:
a. Grimes even stays healthy
b. Finnegan returns back to form or even if he's the #2 CB, remember he played in the slot.
c. Davis and Taylor progress enough to start.
If they do draft a CB in the 1st, Fuller or Verrett, they're automatically starting day one while Taylor/Finnegan/Davis battle out for #3.
This is a deep draft class. Teams can possibly find better talent in the 2nd-3rd rounds opposed to the latter half of the 1st.