They should certainly be full for St Ledger weekend. DSA is very handy for Doncaster races, and also handy for York. But it's not handy for too much else, despite the big regional population.
The link road will certainly help but the trick for me is to use it to make DSA easier to access by public transport. At the moment there's only a stopping bus to Doncaster, and, I think, a very slow stopping bus to Barnsley. Nothing else. I'm sure it would be used more if it were easier to get to.