The last couple of seasons the highest rated guards ahve almost all fallen out of the first round, will that continue this year? i have no clue, but it wouldn't suprise me if Smiley went at 20 or 35 or 40, depending on team needs and other players dropping.
Why trade down? Because in a deep draft like this you will be getting comparable talent in the top of the 2nd round. 2nd round picks are cheaper, and are under a lot less pressure to perform. PLus, you pick up additional picks in the process, and with more picks comes more chances for unearthing potential starters or contributers.
Why pay a guy "#20 money" when you can pay a guy just as good "#35 money"?
Of course, all that goes out the window if somebody you covet is in your draft spot, then you definately pull the trigger on that player.