Though I don't normally feed the trolls, here's my examples:
Example #1
I use the FaceBook app quite a bit to arrange things with friends, etc. The place where I work blocks FaceBook on their network, including wireless. If I have forgotten to disable my wireless and I start FaceBook it will never update it's information. I then see that, swipe my finger across the top, hit the wireless icon and voila, wireless is off and FaceBook is now updating over 3G. Took me less that 2 seconds and only one swipe. To me that's far more useful than having to exit the FaceBook app, switch screens to where my settings are, hit the wifi line, slide the toggle to off, exit settings, switch back to the screen with FaceBook and relaunch the program.
Example #2
I keep my phone at about 60% brightness all the time. Unfortunately, in my brightly lit lunchroom that setting can make it hard to watch movies. Of course I never remember this BEFORE I start the movie/TV show, nope I realize it at the first night/underground or other dimly lit scene. Again, with SBSettings, it's swipe, touch the brightness icon, slide to full and exit. 2 seconds and I never had to leave my movie. Without it i have to exit the iPod app, switch screens to where my settings are, hit the brightness line, slide it up to 100%, exit settings, switch back to the screen with the iPod app, relaunch the program, pick my movie/tv show out of the list and start watching again.
I agree completely and provided 2 real world examples that each probably happen several times a week.
To answer the original question, I keep 3G & Bluetooth on all the time an only launch wifi at home or when I need it.
What you described is one of the biggest flaws in this stupid phone, that and the mail app. For as much as people drool over the iphone OS, saying its so wonderful and easy to use. They sure make it hard to change a simple setting or two.