Honestly, this is how I view the WRs.
Base 2 WR sets:
Flanker: Harvin
Split End: Baldwin
Base 3 WR sets:
Flanker: Kearse/Rice
Split End: Baldwin
Slot: Harvin
And ofcourse with Kearse, Baldwin, and Harvin you can mix and match to create certain matchups as all three are interchangeable.
Then definately in certain situations you'll see Rice (if he makes the team) more exclusively on 3rd downs and in RZ situations.
A rookie will get snaps but not many if top 4 are healthy, he'll get Michael treatment. And if the team keeps Lockette, I would thing his ST value trumps a rookie waiting for snaps altogether.
Also, it dumb to write off Mathews as an afterthought, we've seen lesser players earn their way onto the team... pretty much NorthDallas40oz described Mathews into being a DJ Hackett/Houshmanzadeh type player. If Rice can't return to form, there's value in that.
My sleeper WR though is Phil Bates... he's a better WR than Lockette, so if he can spend this off-season working on acing his Special Team game and possible add reliable Punt Returner to his resume... I could see him beating out Lockette or a late round rook.