Currently connman does not support the situation with point-to-point links when there is no default gateway, instead the interface is the default route.
Unfortunately there's (at least) one bug:
1. Connect to 3G with ppp 2. Connect to wifi 3. Disconnect 3G 4. Default gateway for wifi is not set as it should be
This patch was sent upstream but it's still under discussion what's the best aproach to fix this.