Baird wasn't ready to say it was essential (that would be far too committal for a politician, no?), but he did say that in looking to the future, it seems highly likely that public transit will have to go underground in order to satisfy growing ridership levels.
From the Ottawa Citizen:
"By 2017, there just will not be room for any more buses," he said. "If you're not going to build a tunnel, you have to explain how you get that many buses or rail through the downtown core."
"The city's come forward with a plan," he said. "After the federal announcement, they said they would cut the cost to fit. We've put our money up, and it will be up to them to deliver the project."
He agreed, however, that the light-rail line is a "very, very big project. It's a huge expense."
At the same time, "I don't know what the expense is on gridlock and quality of life if we don't act on public transit," he said.