If they don't tank they will be very easy pickings in Cinci. Also I would worry about injuries with such a sudden transition of surface.
That`s the biggest issue.It`s easy to get injured by playing on grass and then 3 days later on hard courts.And that goes with the travel issues as well.
Didn`t Djokovic suffer by playing Davis Cup after the Us Open last year?
Why should they risk getting an injury a couple of weeks before the Us Open?They could take a few days off,go to Cinci,get some practice and then start in a week.
If they play in Toronto,they basically would play competitive matches without any real practice.The last thing they would need right now is pulling a muscle and rulling themselves out for the Us Open.