Aha! I got EIGHT "plug" processes now after putting it to sleep in vlasovsoft several times! And I did NOT connect to USB as storage since the last boot...
Actually, maybe the number of "plug" processes is proportional not to the number of sleep/wakeup events, but to the number of insmod/rmmod-ing driver modules executed by usbnet.sh script in vlasovsoft. I.e. maybe usbnet.sh should not just rmmod but also kill the "plug" process? Just a guess... I'll do a bit more testing now.
UPDATE: Ok, the above hypothesis is wrong. The number of "plug" processes is proportional to the number of USB plugin events. (Duh.... we could have guessed it from the name of the "plug" process
UPDATE2: The above result is confirmed. Moreover, by "USB plugin" here is meant not only those for USBNET but also plugin as USB mass storage via KSM.
So, the summary of all this: if you plugin your device to computer via USB either as a mass storage or for usbnet, then you must reboot the device after unplugging it. Or at least reboot every 10th or 15th unplug. Otherwise these "plug" processes will flood the system (each instance is using 2MB of memory, though some of it is shared by all instances) and you will run out of memory.