The only reason 3-5-2 is being discussed, is because Odubajo and Robertson aren't natural defenders, sticking one in a back three would be crazy, particularly as Robertson is the only decent option for left wing back.
And getting him into that role is the main reason for considering going 3-5-2 in the first place.
Bruce has made it fairly obvious that if he needs the services of another centre back, it's going to be Huddlestone.
I disagree, 3-5-2 is being discussed because 4-4-2 is losing games against mediocre or poor (arf) opposition.
Odubajo and Robertson aren't natural defenders, we know that; well, some of us do. Is Hudds? Look at how we defend when Dawson is not available, explain why we should not use the squad.
There are options for LWB and, if Hayden is used, Robbo is one of them. But it's all about options; something Steve Bruce has very recently extolled, with his depth and balence of squad remarks.
You seem to focus on playing to Robbo's strengths, I suppose that might be what Bruce is doing, but who knows? There are other players far more likely to come good with goals, just as likely to click. Change things a bit, be able to recover, if needs be. Play to their feet, left or right. Give players opportunities; they have been known to respond.
Can you not think of another reason to go 3-5-2, or do we have to think like Steve Bruce?