Yes, the i-MiEV has an immobilizer system, but it only requires one transponder in the car. A keyless system requires at least 3 to be functional (one at each front door as well as the one inside). If the fob doesn't automatically unlock the doors, you have to take it out of your pocket to unlock the car, at which point a standard key is more convenient as you have a place to put it that isn't back in your pocket. That was one thing that drove me nuts about the BMW i3 I had on extended test drive a couple of years ago. My friend's LEAF was better, but the big plus is that with a keyed ignition, I don't have to worry about my keys punching holes into the seat (I hang my keys outside of my pocket with the MiEV remote as the anchor inside my pocket. Too many keys for them all to fit in my pocket between work and house keys).
Having driven the i-MiEV with a conventional key, the i3 with a half-baked keyless system, and the fully integrated LEAF, I still prefer the i-MiEV's key, primarily for security. A wireless signal can be spoofed from a short distance away, but you have to gain physical access to the key to replicate it. So, while a skilled thief could spoof my transponder ID, the transponder won't unlock the doors, whereas the wireless signal from a keyless fob can.
Also, the key must physically be in the ignition for the car to operate. I've heard numerous accounts of folks dropping the spouse off and not realizing that they had the key. The person gets to where they are going, and finds they can't start the car because the person they dropped off on the other side of town has the key in their pocket.
Bottom line, I prefer the physical key, but if someone manages to retrofit a push button system into the i-MiEV, I would be impressed.