Perimeter defense and rim protection are the most valuable defensive assets in today's era. In past eras, you could argue for post defense but not really these days. Having four (five, ideally, but that's usually a pipe dream) players who can switch and defend on the perimeter is pretty essential to a top tier defense.
I could perhaps see the argument that, if you have perimeter switchability among four or five of your players on the floor, a wing defense ace is not crucial. That's quite possibly true. The loss of Roberson may be hurting Oklahoma City so much not because they lost a top wing defender but that they lost a wing defender at all, leaving them with only one. Westbrook and Anthony aren't exactly great perimeter defenders and they've struggled to field an adequate defensive replacement.