It was a brilliant idea for the organisation to ban birthdays, it provided a number of benefits for them. Along with the ones already mentioned, these are some more-
Free advertising. Not celebrating gets the JWs noticed.
Control. I he org has been able to weild it's own strength by imposing rules that aren't even biblical.
Individual unworthiness. This is important in a cult
It gets children trained from a young age to be identified as someone weird and different.