How often is Evans going to see the field over Crabtree and/or Boldin? I doubt it's going to be much in his rookie season. I don't think it's worth trading in to the top 10 for that.
So you are taking the short term must get someone who impacts the team right now player? Unfortunately there is not one in this draft. There is not one player that will be drafted and start right away this year.
The 9ers are set at every position right now. What they are looking for is starters for next season and players who can come in as back ups and not make mistakes. But next year will be starters.
The closest position that will maybe come close is the nickle corner. But Culliver and Brock are the starters. And Morris will most likely be the nickle. He played extremely well last year when needed. Cook and Cox are the ones who will be replaced right away by rookies. But next season or in an event of an injury the rookie will get playing time.
For WRs it takes a year for them to develop. Some have good years as rookies, but most are mediocre at best even taken in the top 10. But, wouldn't it be nice to have a WR ready to 1 replace Crabtree when he goes as a FA and 2 have a receiver who can go out there and start if an injury happens again.
As once stated by a former coach all skill players take a year to learn. Other than the QB the further you are from the QB the more years it takes. Safeties take 3 years, CBs 2 years, and WRs 1 year to really start playing well.