Shved can play SG, too, but we still need a SG, right? Shved can actually play 1-3, with 2 or 3 probably being his best positions. If Shved is playing 2/3, having a rookie who can play 1/2 well doesn't seem like too much overlap to me. Besides, our weakest area is outside shooting, and McCollum is an excellent shooter.
As far as filling a need, there's not much difference between drafting McCollum and a pure SG (like Oladipo, Caldwell-Pope, or McLemore... McCollum has a 6'6 wingspan which is respectable for a NBA SG -- and just above Caldwell-Pope's 6'5.5, while a little behind McLemore's 6'7.5). If anything, it's valuable that McCollum is more versatile, as we could also use a good backup PG that has more of a future with the team than JJ or Luke. If Ricky plays 36 mpg and we sign a free agent SG (like Mayo or Redick) who starts and plays 28-34 mpg, McCollum could play 12 mpg at PG backing up Ricky and the rest of his time at SG, allowing Shved to still get plenty of minutes at SG and SF, while we develop both players.