Network scanning should probably occur less frequently.
It's definitely not optimal if it uses more power when not connected versus connected.
Also it's nice to have wifi always on with network scanning because it will then automatically connect if it finds a configured network. (for exemple when moving to school, free wifi access, arriving at home...).
One check every 10-15min seems good enough to me. I've no idea if it's possible to have Network manager do this.
On the nokia n800 I believe it is possible to select the interval between scan for wifi. You can definitely keep the wiki on all the time and have no problem. And that's with a 1500mah battery, a lot of mobile phone also have decent battery life with wifi on.
I guess it's mostly a question of optimization.