You could, but if all goes to plan, you'd make more buying SBD and converting to STEEM. You would get that 10% discount to work in your favor. So you could do what you're suggesting and buy $1 of STEEM and get 1.12 STEEM on Bittrex. Or you could buy $1 of SBD and get 1.14 SBD on Bittrex. Then if you sent the SBD to your Steemit account and did the internal system conversion, it is going to convert it to Steem.

If the pricing right now turned out to be the total average for the 3 days, you would end up with a conversion of 1 SBD buying 1.12 STEEM, because the internal conversion treats SBD equal to dollars.

Meaning $1 USD -> 1.14 SBD -> 1.27 STEEM ... This means you could effectively buy STEEM at a price of $0.78 cents per token, where as if you just buy it, you would be paying $0.89 cents. Obviously, the actual values will turn out different as this all works on 3 day averages. This would be something like a 10% discount to the price of STEEM using this method.