A couple of points spring to mind:
1) public transit is not a very big advantage. It is useful only in the handful of US cities that have decent public transport. In Paris, one of the great public transport cities in the world, google still doesn't provide public transport information. In 2012. Apple's solution, to rely on 3rd party devs for this has every chance if providing a solution to iPhone users before Google.
2) But even assuming you're one if the lucky few for whom Google provided public transport information, you can still use Google maps from the browser. You can even add an icon in Springboard for it, giving you full access to google itineraries. The only thing that has really been lost is street view.
In my experience, Google Maps' Transit coverage is far far greater than "a handful of U.S. cities". It's worked everywhere I've been in Japan, in Singapore, Hong Kong, Sweden, London, Athens...
The beauty of Google's Transit compared to apps maintained by the transit companies themselves is that it supports cross-company travel.
Cross company travel may not be much of an issue in the longer run, as the transit apps are not restricted to be offered by the companies themselves, so the apps offering an integrated view should win out.
Hoever, planning e.g. a trip from London to Paris, with local transit on both sides, may be difficult.
Could it not be said that, "Turn-by-turn is not a very big advantage. It is only useful for the handful of people who are too poor not to have a navigation system built into their car?" It seems like the divide between the "iOS 6 maps suck!" crowd and the "iOS 6 maps are awesome!" crowd is whether a private car or public transportation is the primary method of getting around.
I don't think the iOS 6 Maps are too bad, walking directions have been good. I use public transport heavily but Google's public transport stuff didn't work for my area of England, so you don't miss what you never had.
2) But even assuming you're one if the lucky few for whom Google provided public transport information, you can still use Google maps from the browser. You can even add an icon in Springboard for it, giving you full access to google itineraries. The only thing that has really been lost is street view.