The ability to restart adbd using tcpip transport can vary from device to device. Without knowing exactly which device you're using it's very difficult to know the exact cause.
I tested "adb tcpip 5555" on a Galaxy Nexus (takju) running 4.2.1 and found it to work. I also tested on a couple other devices and those did not work.
I managed to get it to work using root access on a Motorola Droid 3. Note that ro.secure=1 and ro.debuggable=0 still. I simply by manually set service.adb.tcp.port" to "5555" as such:
devbox:~/droid3$ adb shell
shell@cdma_solana:/$ getprop | grep 'ro.secure\|debuggable\|qemu'
[ro.secure]: [1]
[ro.debuggable]: [0]
shell@cdma_solana:/$ su -c 'setprop service.adb.tcp.port 5555'
shell@cdma_solana:/$ exit
devbox:~/droid3$ adb tcpip 5555
restarting in TCP mode port: 5555
devbox:~/droid3$ adb connect 172.16.0.14
connected to 172.16.0.14:5555
devbox:~/droid3$ adb -s 172.16.0.14:5555 shell
shell@cdma_solana:/$
Although I specified 5555 when running "adb tcpip" it gets ignored by the device. It seems that this particular device does not have the required access to set the "service.adb.tcp.port" property without root.
Similarly, you can go back to USB as such:
shell@cdma_solana:/$ getprop | grep adb
[persist.service.adb.enable]: [1]
[ro.sys.atvc_allow_all_adb]: [0]
[persist.adb.tcp.port]: []
[init.svc.adbd]: [running]
[service.adb.tcp.port]: [5555]
shell@cdma_solana:/$ su -c 'setprop service.adb.tcp.port ""'
shell@cdma_solana:/$ exit
devbox:~/droid3$ adb -s 172.16.0.14:5555 usb
restarting in USB mode
devbox:~/droid3$ adb shell
shell@cdma_solana:/$
The reasons that this might not work on other devices vary.
On the Galaxy Nexus I tested, the shell user is able to set the "service.adb.tcp.port" property. It's not clear why this differs at this time.
Interestingly it seems the droid3 will not continue to listen on USB if you enable TCPIP. However, the Galaxy Nexus will.
Hope this helps..
PS. See also: How can I connect to Android with ADB over TCP?